TITLE: METHOD AND DEVICE FOR USE IN DC PARAMETRIC TESTS
FIELD OF THE INVENTION
The present invention relates generally to electronic chips and devices, and more particularly, to a method and device suitable for use in performing DC parametric tests.
BACKGROUND OF THE INVENTION
Recent years have seen a rapidly increasing demand for highly integrated mixed- signal integrated circuits (IC's). This demand is mostly driven by the ever-expanding communications industry. However, as the level of integration increases, more and more mixed-signal components are becoming buried deep inside large amounts of digital circuitry without any external I/O access. This creates a difficult problem for initial device and circuit characterisation and diagnosis, as well as during a production test. For example, to measure the bias current for a high precision ADC circuit requires some form of external access. However, the access mechanism, such as a test bus, can introduce additional noise from off-chip sources.
Typically circuit characterisation includes the determination of the electrical characteristics of a circuit such as for example measuring the input/output impedance of an amplifier circuit, or finding the voltage transfer characteristics of an amplifier circuit or transistor device amongst others.
One particular area of IC testing that is being affected is the DC parametric tests. These tests are typically conducted to characterise a wide variety of mixed-signal circuits such as Analog-to-Digital Converters (ADCs), PLLs and bias networks. Also, these tests are used in digital test applications such as pad current leakage and IDDQ tests. For example, the pad current leakage test and the IDDQ test are common test techniques for detecting faults in digital ICs.
DC parametric tests are generally classified as one of two types. The first type of DC parametric test involves forcing a voltage at a circuit node while measuring the current that flows into the node. Commonly used method for on-chip current measurements include using device having either a transimpedance amplifier, as shown in figure la), an integrating network as shows in figure lb) or a shunt resistance, as shown in figure lc). For additional information regarding the above mentioned methods, the reader is invited to refer to the following documents:"
1. Teradyne, Inc., "Low Current Ammeter Channel Card", Advanced Mixed- Signal Instrumentation Manual, 1996.
2. C. D. Thompson, S. R. Bernadas, "A Digitally-Corrected 20b Delta- Sigma Modulator", Proc. IEEE International Solid-State Circuits Conference, pp. 194-195, 1994.
3. U.S. Patent No. 5,274,375 issued to Charles D. Thompson December 28, 1993;
4. M. Breten, T. Lehmann, E. Bruun, "Integrating Data Converters for Picoampere Currents from Electrochemical Transducers", Proc. IEEE International Symposium on Circuits and Systems, Vol. 5, pp. 709-712, May 2000. 5. C. B. Wang, J. Todsen, T. Kalthoffi "A Dual Channel 20 Bit Current-Input
A/D Converter for Photo-Sensor Applications", Proc. Southwest Symposium on Mixed-Signal Design, pp. 57-60, 2000.
6. Burr-Brown Product # DDC112
7. J. Kotowski, B. Mclntyre, J. Parry, "Current Sensor IC Provides 9 bit + Sign Result without External Sense Resistor", Proc. IEEE Custom
Integrated Circuits Conference, pp. 35-38, 1998;
8. U.S. Patent No. 5,867,054 issued to Jeffrey P. Kotowski February 2, 1999;
9. National Semiconductor Product # LM3814
The contents of the above documents are hereby incorporated by reference.
A deficiency of devices of the type described above is that they involves the use of elaborate Analog-to-Digital Converters (ADCs) with trimmed components, which
makes these devices expensive and relatively non-scalable for on-chip implementation. Another deficiency of devices of the type described above is that they make use of op-amps (operational amplifiers) which also makes them relatively non-scalable for on-chip implementation. Generally, the size of the op-amp circuit does not shrink to the same extent as the size of logic circuits do as IC technology advances.
The second type of DC parametric test involves forcing a known current into a circuit node while measuring the voltage at the node!
A deficiency of commonly used on-chip current sources is that they generally suffer from low output resistance and shifts in current levels due to process variation. Such current sources are described in W. Sansen et al., "A CMOS Temperature- Compensated Current Reference", IEEE Journal of Solid-State Circuits, Vol. 23, pp. 821-824, June 1988 and in H. J. Oguey et al., "CMOS Current Reference Without Resistance", IEEE Journal of Solid-State Circuits, Vol. 32, pp. 1132-1135, July 1997 whose contents are herein incorporated by reference. Other on-chip current source implementations, of the type described in Burr-Brown Corporation, "Dual Current Source/Current Sink", REF200 (Datasheet), Oct. 1993 and in U.S. Patent No. 4,792,748 issued to David M. Thomas et al. in December 20, 1998, can generally achieve good current accuracy but require laser-trimmed on-chip resistors, which is costly when multiple measurement units are required on a single chip. The contents of the above documents are hereby incorporated by reference.
In the context of the above, there is a need in the industry to provide a method and device for use in performing DC parametric tests that alleviates at least in part problems associated with the existing devices and methods.
SUMMARY OF THE INVENTION
In accordance with a first broad aspect, the invention provides a circuit device suitable for use in performing a DC parametric test on an external load. The circuit device
includes an input suitable for receiving a forcing parameter signal, an output suitable for releasing to the external load a signal approximating the forcing parameter signal, a first circuit segment and a second circuit segment. The first circuit segment is located between the input and the output and includes a search entity, an intermediate voltage point and an internal load between the intermediate voltage point and the output of the circuit device. The second circuit segment is connected in a feedback arrangement with the first circuit segment. The second circuit segment provides the search entity in the first circuit segment with a first voltage signal indicative of the voltage at the output of the circuit device. The search entity is adapted for generating a second voltage signal on the basis of the forcing parameter signal and the first voltage signal received from the second segment and for applying the second voltage signal to the intermediate voltage point. The application of the second voltage signal to the intermediate voltage point causes a change in either one of the voltage signal or the current signal at the output of the circuit device such that, at equilibrium, a signal approximating the forcing parameter signal is caused at the output.
In accordance with another broad aspect, the invention provides a circuit for performing a DC parameter test on an external load. The circuit comprises a circuit input, a circuit output, a search unit, a circuit module having digital-to-analog conversion functionality and load functionality and an analog-to-digital converter (ADC). The circuit input is for receiving a forcing parameter signal. The circuit output is for connection to the external load. The search unit has a first input connected to the circuit input and has a second input and an output. The circuit module has digital-to-analog conversion functionality and load functionality and is connected between the output of the search unit and the circuit output. The analog-to- digital converter (ADC) is connected between the circuit output and the second input of the search unit. The search unit is adapted to generate a digital target voltage at its output on the basis of a voltage at the circuit output and the forcing parameter signal whereby a signal approximating the forcing parameter signal is derived at the circuit output.
Advantageously, the above-described circuit can be implemented using some digital logic as permitted by the use of ADCs and DACs. The digital logic allows taking advantage of the down-scaling of digital integrated circuit technology and facilitates the on-chip implementation of such devices.
In accordance with a specific example implementation, the forcing parameter signal is a forced voltage signal.
In accordance with an alternative specific example implementation, the forcing parameter signal is a forced current signal.
Specific examples of implementation may make use of a forcing parameter signal that is in either one of an analog format or digital format.
In a specific example of implementation, the circuit module having digital-to-analog conversion functionality and load functionality includes a low-pass filter module one side of which is connected to the circuit output and a pulse generator module connected between the output of the search unit and another side of the low-pass filter module.
In an alternate specific example of implementation, the circuit module having digital- to-analog conversion functionality and load functionality includes an internal load one side of which is connected to the circuit output and a digital-to-analog converter (DAC) connected between the output of the search unit and another side of the internal load.
In accordance with a first non-limiting implementation, the internal load in the circuit device is a linear non-inverting load. Such a linear non-inverting load may include one or more linear resistor elements, RC (resistor/capacitor) circuit elements and any other suitable linear analog circuit having linear non-inverting properties. Such linear analog circuits having linear non-inverting properties may include non-linear components arranged in such a manner to produce a linear non-inverting load. Such
non-linear components may include for example PMOS circuits, NMOS circuits, CMOS circuits, BJT circuits, BiCMOS circuits, JFET circuits and MESFET circuits.
In accordance with a second non-limiting implementation, the internal load in the circuit device is a non-linear and inverting load. Such a non-linear and inverting load may include one or more MOS elements, BJT circuits, JFET circuits, diode circuits, MESFET or BiCMOS circuits. MOS configurations including a CMOS circuit, a PMOS circuit and NMOS circuit may also be used.
In accordance with a third non-limiting implementation, the internal load in the circuit device is a non-linear and non-inverting load. Such a non-linear and non-inverting load may include one or more MOS elements, BJT circuits, JFET circuits, diode circuits, MESFET or BiCMOS circuits.
In accordance with a fourth non-limiting implementation, the internal load in the circuit device is a linear and inverting load. Such a linear and inverting load may include one or more MOS elements, BJT circuits, JFET circuits, MESFET or BiCMOS circuits.
Advantageously, by using a non-linear internal load, larger changes in current at the output can be established for a smaller corresponding change in voltage applied at the output. In addition, the downscaling of digital circuit will limit the power supply to lower voltage levels, which in turn will limit the output voltage range of analog circuits. Consequently, circuits of the type described above making use of a non- linear internal load will be affected to a lesser extent by reductions in power supply voltages.
In a specific non-limiting example of implementation where the forcing parameter signal is a forced voltage signal, the search unit includes a digital comparator, a digital integrator module and an output. The digital comparator is for generating a digital difference voltage signal dependent on the difference between the forced voltage signal and the digital approximation of the voltage signal at the output of the circuit
device. The digital integrator module is adapted for processing the digital difference voltage signal to derive the digital target voltage. The digital target voltage is released at the output of the search unit for processing by the digital-to-analog converter module. Where the forcing parameter signal is an analog signal, the search unit further includes an analogue-to-digital converter module for processing the forced voltage signal to generate a corresponding digital forced voltage signal. The digital forced voltage signal is then provided to the digital comparator.
In accordance with a specific example of implementation, the analog-to-digital converter module in the circuit device includes an analog comparator, a digital integrator and a feedback circuit. The analog comparator receives a signal indicative of the voltage at the output and a tracking voltage and generates a difference signal on the basis of the signals received. The digital integrator receives the difference signal and generates successive digital approximations of the voltage signal at the output of the circuit device. The feedback circuit processes the successive digital approximations of the voltage signal to generate the tracking voltage and provide the tracking voltage to the analog comparator. In a non-limiting implementation, the feedback circuit includes a digital-to-analog converter module. In accordance with an alternative specific example of implementation, the digital integrator in the analog-to- digital converter module is replaced by a successive-approximation circuit (SAR) module. For additional information regarding successive-approximation circuits (SAR), the reader is invited to consult D. A. Johns, K. Martin, Analog Integrated Circuit Design, John Wiley & Sons, Inc., pp. 492-493, 1997. The content of this document is hereby incorporated by reference.
In accordance with another broad aspect, the invention provides a system for providing a current measurement suitable for use in performing a DC parametric test on an external load. The system includes a voltage generating circuit device and a current measurement circuit. The voltage generating circuit device includes an input for receiving a signal indicative of a forced voltage, an output suitable for releasing to the external load a signal approximating the forced voltage, a first circuit segment and a second circuit segment. The first circuit segment is between the input and the
output and includes a search unit, an intermediate voltage point and an internal load between the intermediate voltage point and the output. The second circuit segment is connected in a feedback arrangement with the first circuit segment. The second circuit segment provides the search unit in the first circuit segment with a first voltage signal indicative of the voltage at the output. The search unit is adapted for generating a second voltage signal on the basis of the signal indicative of the forced voltage and the first voltage signal received from the second segment and applying the second voltage signal to the intermediate voltage point. The application of the second voltage to the intermediate voltage point causes a change in either one of the voltage signal or the current signal at the output such that, at equilibrium, a voltage approximating the forced voltage is caused at the output. The current measurement circuit includes a first input for receiving a first signal derived from the second voltage signal, a second input for receiving a second signal derived from the signal indicative of the forced voltage signal, search logic and an output. The search logic derives a certain current measurement on the basis of the first signal and the second signal. The certain current measurement is released at the output of the current measurement circuit. When the system is in equilibrium, the certain current measurement is indicative of an approximation of measurement of the current flowing at the output of the voltage generating circuit when the signal indicative of the forced voltage is applied to the output of the voltage generating circuit.
In accordance with a non-limiting example, the first signal derived from the second voltage signal includes a digital representation of the second voltage signal and the second signal derived from the signal indicative of the forced voltage includes a digital approximation of the signal indicative of the forced voltage.
In accordance with another broad aspect, the invention provides a system for providing a current measurement for use in performing a DC parametric test on an external load. The system includes a voltage generating circuit device and a current measurement circuit. The voltage generating circuit device includes a circuit input, a circuit output, a search unit, a circuit module having digital-to-analog conversion functionality and load functionality and an analog-to-digital converter (ADC). The
circuit input is for receiving a signal indicative of a forced voltage. The circuit output is for connection to the external load. The search unit has a first input connected to the circuit input and has a second input and an output. The circuit module having digital-to-analog conversion functionality and load functionality is connected between the output of the search unit and the circuit output. The analog-to-digital converter (ADC) is connected between the circuit output and the second input of the search unit. The search unit is adapted to generate a digital target voltage at its output on the basis of a voltage at the circuit output and the signal indicative of the forced voltage whereby a signal approximating the forced voltage is applied at the circuit output. The current measurement circuit includes a first input for receiving a first signal derived from the digital target voltage, a second input for receiving a second signal derived from the forced voltage signal and a search logic module. The search logic module is coupled to the first and second inputs and derives a certain current measurement on the basis of the first signal derived from the digital target voltage and the second signal derived from the forced voltage signal and a search logic module. When the system is in equilibrium, the certain current measurement is indicative of an approximation of measurement of the current flowing at the output of the voltage generating circuit when the signal approximating the forced voltage is applied to the output of the voltage generating circuit. The certain current measurement is released at an output of the current measurement circuit.
In a specific implementation, the circuit module having digital-to-analog conversion functionality and load functionality includes a low-pass filter module one side of which is connected to the circuit output and a pulse generator module connected between the output of the search unit and another side of the low-pass filter module.
In an alternative specific implementation, the circuit module having digital-to-analog conversion functionality and load functionality includes an internal load one side of which is connected to the circuit output and a digital-to-analog converter (DAC) connected between the output of the search unit and another side of the internal load.
In accordance with a specific implementation, the search logic of the current measurement circuit includes a data structure having a plurality of entries, each entry
providing a mapping between a data element conveying a given target voltage and data element conveying a given the forced voltage to a corresponding current measurement. The data structure may be stored on any suitable memory unit such as a RAM, ROM, PROM, EPROM and EEPROM. In a specific non-limiting implementation, the data structure is stored on a RAM device.
Advantageously, the search logic captures the DC transfer characteristic of the internal load at different current levels. Once known, the transfer characteristic of the internal load can be used indirectly to determine the value of an unknown current level at the output of the circuit device when the output is connect to an external load.
In accordance with yet another broad aspect, the invention provides a current generating circuit device suitable for use in performing a DC parametric test on an external load. The circuit device includes a circuit input, a circuit output, a search unit, a circuit module having digital-to-analog conversion functionality and load functionality and an analog-to-digital converter (ADC). The circuit input is for receiving a signal indicative of a forced current. The circuit output is for connection to the external load. The search unit has a first input connected to the circuit input and has a second input and an output. The circuit module having digital-to-analog conversion functionality and load functionality' is connected between the output of the search unit and the circuit output. The analog-to-digital converter (ADC) is connected between the circuit output and the second input of the search unit. The search unit is adapted to generate a digital target voltage at its output on the basis of a voltage at the circuit output and the signal indicative of a forced current whereby a signal approximating the signal indicative of a forced current is derived at the circuit output.
In accordance with a specific example, the circuit module having digital-to-analog conversion functionality and load functionality includes a low-pass filter module one side of which is connected to the circuit output and a pulse generator module connected between the output of the search unit and another side of the low-pass filter module.
In accordance with an alternate example, the circuit module having digital-to-analog conversion functionality and load functionality includes an internal load one side of which is connected to the circuit output and a digital-to-analog converter (DAC) connected between the output of the search unit and another side of the internal load.
In accordance with a specific example of implementation, the search unit includes a data structure having a plurality of entries, each entry providing a mapping between:
1. a data element conveying the voltage signal at the output of the circuit device; and
2. a data element conveying a given forced current and a data element conveying a target voltage.
In a non-limiting implementation, the data structure may be stored on any suitable memory unit such as, but not limited to, a RAM, ROM, PROM, EPROM and EEPROM. In a specific embodiment, the memory unit is stored on a RAM.
In accordance with an alternative implementation, the search unit provides a data structure having a plurality of entries, each entry providing a mapping between: 1. data elements conveying voltage signals at the output and a data element conveying a given forced current signal; and 2. data elements conveying target voltage values.
These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
Figures la, lb and lc show examples of prior art circuit devices for on-chip current measurements; Figures 2a and 2b show circuit devices suitable for use in performing a DC parametric test on an external load in accordance with non-limiting examples of implementation of the invention;
Figure 3 is a graph showing the DC characteristics of internal load of the circuit device shown in figure 2b in accordance with a non-limiting example of implementation of the invention; Figure 4a and 4b show specific example of implementation of the internal load of the circuit device shown in figure 2b in accordance with a non-limiting example of implementation of the invention; Figure 5 is a graph showing the DC characteristics of external load R2 in accordance with a non-limiting example of implementation of the invention; Figure 6 is a graph showing on a same graph the load lines of internal load Ri and the load line of internal load R2 depicted in figures 3 and 5 when the loads
Ri. and R2 are connected in series; Figure 7 is a graph showing the location of the desired voltage to apply to the intermediate voltage point VDAC shown in figure 2b of the drawings for a desired forcing parameter voltage signal Nforce in accordance with a specific example of implementation of the invention;
Figure 8 shows the circuit device of figure 2b adapted for searching for a voltage to apply to intermediate voltage point VDAC for a desired forcing parameter voltage signal Nforce in accordance with a non-limiting example of implementation of the invention; Figure 9 is a graph showing the DC characteristics of internal load Ri of the circuit device shown in figure 2b and showing the location of the current measurements in accordance with a non-limiting example of implementation of the invention; Figure 10 is a graph illustrating a function mapping a forcing parameter signal Nforce and the voltage to apply to intermediate voltage point VDAC to a current value at the output of the circuit device shown in figure 8 in accordance with a non-limiting example of implementation of the invention;
Figure 11 shows the circuit device of figure 8 adapted for further providing a current measurement value in accordance with a non-limiting example of
• implementation of the invention;
Figure 12 shows a calibration circuit for calibrating the circuit device shown in figure 8 in accordance with a non-limiting example of implementation of the invention;
Figure 13 is a graph showing a plot of k DAC VS. Ioutin accordance with a non-limiting example of implementation of the invention; Figure 14 shows a test circuit for calibrating the circuit device shown in figure 8 including a test current reference Itest in accordance with a non-limiting example of implementation of the invention; Figure 15 shows an alternate calibration circuit for calibrating the circuit device shown in figure 8 in accordance with a non-limiting example of implementation of the invention; Figure 16 shows the circuit device of figure 2b adapted for searching for a voltage to apply to intermediate voltage point VDAC for a desired forcing parameter current signal Iforce in accordance with a non-limiting example of implementation of the invention; Figure 17 is a graph showing the location of a desired voltage to apply to intermediate voltage point VADC for a desired forcing parameter current signal Iforce in accordance with a non-limiting example of implementation of the invention; Figure 18 shows the circuit device of figure 2b adapted for forcing a voltage at output NADC for a desired forcing parameter voltage signal Iforce in accordance with a non-limiting example of implementation of the invention;
Figure 19 is a graph showing the location of a desired voltage to apply to output VADC for a desired forcing parameter current signal Iout=Iforce in accordance with a non-limiting example of implementation of the invention; Figure 20 is a graph showing the DC characteristics of internal load Ri of the circuit device shown in figure 2b and showing the location of the voltage measurements for I0ut=If0rce in accordance with a non-limiting example of implementation of the invention;
Figure 21 is a graph illustrating a function "H" mapping a forcing parameter signal
Iforce and the voltage to apply to intermediate voltage point VDAC to a voltage value at the output of the circuit device shown in figure 16 in accordance with a non-limiting example of implementation of the invention;
Figure 22 is a graph illustrating a function "H"1" mapping forcing parameter signal Iforce and to a voltage value at the output of the circuit device shown in figure 16 to a voltage to apply to intermediate voltage point VDAC accordance with a non-limiting example of implementation of the invention;
Figures 23 a), b), c) and d) show four alternate specific examples of implementation of the circuit device of figure 2b adapted for searching for a voltage to apply to intermediate voltage point VDAC or VADC for a desired forcing parameter voltage signal Iforce in accordance with non-limiting examples of implementation of the invention;
Figures 24 and 25 are graphs showing on a same plot the load lines of internal load Ri. and the load line of internal load R2 when the loads Ri and R2 are connected in series in accordance with a non-limiting example of implementation of the invention; Figure 26 shows an alternate specific example of implementation of the circuit device of figure 2b adapted for searching for a voltage to apply to intermediate voltage point VDAC for a desired forcing parameter voltage signal Iforce in accordance with a non-limiting example of implementation of the invention; Figure 27 and 28 show portions of calibrating circuits for calibrating the circuit device shown in figure 23 in accordance with a non-limiting example of implementation of the invention;
Figure 29 shows the circuit device of figure 2b adapted for searching for a voltage to apply to intermediate voltage point VDAC for a desired forcing parameter voltage signal Nforce in accordance with a non-limiting example of implementation of the invention where the internal load R\ is an inverting load;
1.5
Figure 30 shows a modified version of the calibration circuit of figure 15 adapted for an inverting internal load Ri in accordance with a non-limiting example of implementation of the invention; Figure 31 shows a modified version of the calibration circuit of figure 18 adapted for an inverting internal load Ri in accordance with a non-limiting example of implementation of the invention; Figure 32 shows a modified version of the calibration circuit of figure 23a) adapted for an inverting internal load Ri in accordance with a non-limiting example of implementation of the invention; Figure 33 shows a modified version of the calibration circuit of figure 23b) adapted for an inverting internal load Ri in accordance with a non-limiting example of implementation of the invention; Figure 34 shows a modified version of the calibration circuit of figure 23 c) adapted for an inverting internal load Ri in accordance with a non-limiting ' example of implementation of the invention;
Figure 35 shows the circuit device of figure 2b adapted for forcing voltage VDAC on the basis of a forcing parameter including a linear resistive internal load
Ri in accordance with a non-limiting example of implementation of the invention; Figure 36 shows the circuit device of figure 2b adapted for forcing voltage VADC on the basis of a forcing parameter including a linear resistive internal load
Ri in accordance with another non-limiting example of implementation of the invention; Figure 37 shows the circuit device of figure 11 where internal load Ri is a linear resistive load in accordance with a non-limiting example of implementation of the invention; Figures 38 and 39 show calibration circuits for a a linear resistive internal load Rj. suitable for use in calibrating the circuit devices of figure 36 and 37 in accordance with non-limiting examples of implementation of the invention;
Figures 40-62 show various embodiments of the circuit device of figure 2b in accordance with non-limiting examples of implementation of the invention;
Figures 63-66 show an alternative configuration of a circuit device suitable for use in performing a DC parametric test on an external load in accordance with a non-limiting example of implementation of the invention;
Figure 67 shows another alternative configuration of a circuit device suitable for use in performing a DC parametric test on an external load in accordance with another non-limiting example of implementation of the invention.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
DETAILED DESCRIPTION
With reference to Fig. 2a and 2b, there is shown a general configuration of a system 200 suitable for use in performing a DC parametric test and an unknown external load 206 representing a circuit under test.
The system 200 includes an input 202 for receiving a forcing parameter signal, an output 204 suitable for releasing to an external load 206 a signal approximating the forcing parameter signal. The system also includes a first circuit segment between the input 202 and the output 204 and a second circuit segment connected in a feedback arrangement with the first circuit segment.
As shown in figure 2a, the first circuit segment includes a search entity 199, an intermediate voltage point 214 and an internal load 216 between the intermediate voltage point and the output 204. The second circuit segment provides the search entity 199 in the first circuit segment with a signal indicative of the voltage at the output 204. The search entity 199 is adapted for generating a second voltage signal on the basis of the forcing parameter signal received at input 202 and the first voltage
signal received from the second segment and to apply the second voltage signal to the intermediate voltage point 214. The application of the second voltage signal to the intermediate voltage point 214 causes a change in either one of the voltage signal or the current signal at the output 204 such that a signal approximating the forcing parameter signal is caused at the output 204.
It will be appreciated to the person skilled in the art that the search entity 199 may be adapted for processing and generating signals in either digital format or analog format or a combination of both without detracting from the spirit of the invention. For example, the forcing parameter signal applied at input 202 and the second voltage signal released by the search entity 199 may also be in either digital or analog format.
In a specific configuration shown in figure 2b, the search entity 199 is shown including a search unit 208, an analog-to-digital converter module (ADC) 212 and a digital-to-analog converter module (DAC) 210. In this configuration, the second circuit segment includes the analog-to-digital converter module (ADC) 212 for processing the voltage at the output 214 to generate a digital approximation 85 of the voltage signal at the output 204. The first circuit segment includes the search unit 208, the intermediate voltage point 214, the internal load 216 between the intermediate voltage point 214 and the output 204 and the digital-to-analog converter module (DAC) 210. The search unit 208 is for generating a digital target voltage 87 on the basis the digital approximation of the voltage signal at the output 204 and the forcing parameter signal at input 202. The digital-to-analog converter module 210 is between the search unit 208 and the intermediate voltage point 214 and is adapted for processing the digital target voltage 87 to generate a second voltage signal and applying the second voltage signal to the intermediate voltage point 214. The application of the second voltage signal to the intermediate voltage point 214 causes a change in either one of the voltage signal and the current signal at the output 204 such that, at equilibrium, a signal approximating the forcing parameter signal is caused at the output 204.
In yet another configuration (not shown in the figures), the first path includes a search unit and a circuit module having digital-to-analog conversion functionality and load functionality. The circuit module is connected between the output of the search unit and the circuit output. A non-limiting example of a circuit module having digital-to- analog conversion functionality and load functionality is shown in figures 46A and 46B which will be described in greater detail further on in the specification. Another example of a circuit module having digital-to-analog conversion functionality and load functionality is a circuit module including digital-to-analog converter module (DAC) 210 and internal load 216 connected as depicted in figure 2b.
With reference to figure 2b, when the system 200 is in use, a first terminal of external load R 206 is connected to output 204. For the purpose of simplicity, it is assumed that the other terminal of external load R2 206 is connected to a fixed voltage 220. The resistance of external load R2 206 can be derived from the voltage VADC at output 204, from the voltage at 220 and by the current Iout flowing at output 204.
The input 202 allows receiving either a forcing voltage value or a forcing current value. In embodiments where the input 202 is for receiving a forcing voltage, the set of functional elements allows forcing the voltage VADC at output 204 to a required voltage value at the external load R2 206 using a search algorithm implemented by search unit 208. The search unit 208 controls the voltage at intermediate voltage point 214, which is input to the Digital-to-Analog-Converter (DAC) 210, such that the desired voltage VADC at output 204 is set as desired. In embodiments where the input 202 is for receiving a forcing current, the set of functional elements allows forcing the current Iout at output 204 to a required current value at the external load R2 206.
Depending on the forcing parameter (the voltage VADC or Iout at output 204), different searching algorithms are implemented by the search unit 208. The algorithms implemented by search unit 208 are described in greater detail herein below.
The resistance of the internal load Ri 216 depends on both terminal voltage VDAC at point 214 and voltage VADC at output 204. The Analog-to-Digital-Converter (ADC)
212 has an input coupled to output 204 and an output coupled to the search unit 208. The ADC 212 has essentially an infinite resistance in the operating range of the system 200. As such the ADC 212 does not draw current and consequently, the current at output 204 flowing in external load 206 also flows into internal load Ri 216.
Internal Load Ri 216
In a specific implementation, the internal load Ri 216 is a non-linear resistor device whose resistance depends on both terminal voltages VDAC and VADC- AS a result, Iout is dependent on both VDAC and VADC A non-limiting example of the DC characteristics of internal load Ri 216 is shown in Figure 3 of the drawings.
Positive Resistance Property
From Figure 3, it can be observed that for any particular voltage VDAC at intermediate voltage point, current Iout at 204 increases when the voltage VADC at 204 increases. Mathematically, this can be expressed by the following:
Equation 1
ADC > 0 for all voltage values VDAC dl_
An internal load Ri 216 is said to have a "positive resistance" if the condition in equation 1 is satisfied. Two non-limiting examples of implementations of such a load element are shown in Figures 4(a) and 4(b). Figure 4(a) shows a linear resistor and Figure 4(b) shows a CMOS inverter. For either one of these elements, the current Iout at 204 increases with voltage VADC if voltage VDAC is fixed at any voltage. Hence the examples of load elements shown in Figures 4(a) and 4(b) follow the relationship in equation 1.
For the purpose of simplicity, only elements with a positive resistance will be used for internal load Ri 216. Therefore, in all the analysis that follows, Ri will be assumed to have a positive resistance. It will be readily apparent to the person skilled in the art in
light of this description that an implementation where Ri has a non-positive resistance can be implemented without detracting from the spirit of the invention.
Non-Inverting Property
If the voltages at the intermediate voltage point 214 correspond to VDAC voltages in Figure 3 as VDAC3>VDAC2>VDACI5 then voltage VADC will increase with voltage VDAC if lout is held at any constant value. Mathematically, this can be expressed by the following:
Equation 2
^^ > 0 for all Iout dVDAC
A load element Ri 216 is said to be "non-inverting" if the condition in equation 2 is satisfied. An example of a non-inverting load Ri 216 is shown in Figure 4 (a). It will be readily apparent to the person skilled in the art that if current Iout at output 204 is fixed at any value, there will be a constant potential difference between VADC and VDAC- Therefore, VADC increases with VDAC- The condition in equation 2 is satisfied.
Alternatively, load element Ri 216 may be "inverting" if it satisfies the following condition. If the voltages shown in Fig. 3 at the intermediate voltage point 214 as VDAC3<VDAC2<VDACI, then VADC will increase while VDAC decreases (and vice versa) if lout is held constant. Mathematically, this can be expressed by the following:
Equation 3
^∞ < 0 for all lout
WDAC
A load element Ri 216 is said to be "inverting" if the condition in equation 3 is satisfied. An example of an inverting Ri is shown in Figure 4(b). For a constant Iout, the VADC increases while VDAC decreases, and vice versa.
Depending on whether an inverting or a non-inverting internal load Ri 216 is implemented in the system 200 (shown in figures 2a and 2b), slightly different search algorithms have to be implemented by the search unit 208. The search algorithms implemented by the search unit 208 will first be described with reference to an non- inverting internal load Ri 216. The algorithm modifications for an inverting-type load Ri 216 will be described later on in the specification.
External Load R2 206
With reference to figures 2a and 2b of the drawings, the current Iout flowing from external load R2 at output 204 is dependent on the voltage VADC at the output 204. In most microelectronic circuits, whenever a current is pumped into a node, the node voltage increases. In a specific example of implementation, the system 200 is designed with the assumption that current Iout decreases as VADC increases (i.e., VADC increases when a positive current is pumped into R2). Mathematically, this constraint can be expressed as
Equation 4 dl_
< 0 dV A,DC
An example DC characteristic of external load R2 206 that follows equation 4 is shown in Figure 5. For the purpose of simplicity, R2 is assumed to follow the property defined by equation 4. It will be readily apparent to the person skilled in the art in light of this description that an implementation where R2 does not follow the property defined by equation 4 can be implemented without detracting from the spirit of the invention.
Series-connected Loads
When the system 200 is in operation, the internal load Ri 216 is connected in series with external load R2 206 as shown in Figures 2a and 2b. As a result, the load currents (Iout's) through the load Ri 216 and load R2 206 are equal. The DC bias points can be found by overlapping the DC plots of Figure 3 and Figure 5 to form a load-line plot shown in Figure 6.
It can be seen from Figure 6 that for a voltage VDAC at intermediate voltage point 214, there is only one valid VADC and Iout at output 204 that can satisfy the equilibrium condition. Thus VADC and Iout are functions of voltage VDAC- Mathematically, this relationship can be expressed as two simultaneous equations:
Equation 5
VADC = ft (VDAC)
Equation 6 lout = £_(VDAC)
The relationships in equations 5 and 6 imply that it is possible to force either a voltage (VADC) or a current (Iout) at output 204 by establishing a corresponding voltage VDAC at the intermediate voltage point 214.
Force- Voltage-Measure-Current Algorithm
In a first example of implementation of the invention, the input 202 (shown in figures 2a and 2b) is adapted for receiving a forcing parameter signal indicative of a forced voltage signal Vf0rce- In such an implementation, the system is a voltage generating circuit and the search unit 208 is said to implement a force voltage algorithm.
The Search Algorithm
An objective of the force- voltage algorithm is to vary the voltage VDAC at the intermediate voltage point 214 such that the voltage at the output (VADC) 204 will be set to approximate the desired forcing voltage Vforce. We will refer to this desired VDAC voltage as V*DAC-
Figure 7 shows a graphical view of the search process used to obtain V DAC- For a particular Vforce applied at input 202, the operating current Iout at output 204 corresponds to the intersection of a vertical line representing VADC=Nforce and the load line for R2. This point of intersection is labelled as point A 71 in figure 7. The search algorithm adjusts VDAC at the intermediate voltage point 214 such that the load line for load Ri 216 intersects point A 71. The corresponding VDAC is then V DAC-
Figure 8 shows the circuit device of figure 2b adapted for searching for a voltage to apply to intermediate voltage point VDAC for a desired forcing parameter voltage signal Vforce in accordance with a non-limiting example of implementation of the invention.
As shown in figure 8, a forced voltage signal Vforce is applied to input 202. The search unit 208 includes an ADC 80, a digital comparator 82, a digital integrator module 83 and an output for releasing a digital target voltage koAC 87. The ADC 80 processes the forced voltage signal Vforce to generated a digital forced voltage signal kpoRCE 84. The digital comparator 82 is for generating a digital difference voltage signal dependent on the difference between the digital forced voltage signal 84 and the digital approximation 85 of the voltage signal at the output 204 of the circuit device. The digital integrator module 83 is adapted for processing the digital difference voltage signal to derive the digital target voltage kDAc 87. The digital target voltage kϋAc 87 is released at the output of the search unit 208 for processing by the digital- to-analog converter module 210. Where the forcing parameter signal is a digital signal, the ADC 80 may be omitted from the system shown figure 8.
Mathematically, kforcβ 84 can be expressed as follows:
Equation 7
_ JΑ force\ _ "for force y γ r LSB- ADC v LSB- ADC
where Q[x] is the quantizer function of the ADC 80, VLSB-ADC is the LSB voltage of 5 the ADC 80 and vforce is the quantized Vforce aplied at 202. Also, the digitized value of VDAC at intermediate point 214 is denoted by koAC at point 87 in figure 8.
It will be readily apparent that for any voltage x and its quantized value Q[x], the difference is no bigger than half the LSB voltage of the ADC 80 (VLSB-ADC)- This 0 voltage difference is negligible when ADC 80 has a sufficiently small quantization step. Therefore, for the purpose of simplicity of the description, the following assumption have been made:
Equation 8 5 x — Q_x = kx VLSB_ADC
where k is the digital representation of voltage x at the output of the ADC. From equation 8, it can be seen that:
Equation 9 l0" V ' f.orce - V ' f .orce
Therefore, equation 7 becomes:
Equation 10
JL force force τr
" LSB- ADC 5
Similarly, for the ADC 212 at the feedback path of Figure 8,
Equation 11
track jr
* LSB- ADC
where VADC is the voltage at output 204 and ktrack is the digital approximation of the voltage signal VADC at node 85 in figure 8.
Upon equilibrium in the system, VADC= Vforce and VDAC=V*DAC-
Current Measurement
Once the voltage Vforce has been applied, the current Iout at output 204 needs to be obtained in order to obtain the DC characteristics of external load R2 206.
With reference to figure 9, the load lines of internal load Ri 216 are shown with reference to current Iout at output 204. It can be seen from this figure that Iout is dependent on both the voltage VDAC at the intermediate voltage point 214 and on the voltage VADC at the output 204. Mathematically, this can be expressed as follows:
Equation 12 lout = G (VDAC , VADC)
If the voltage VADC at output 204 is kept constant at a voltage which approximates Vforce, current Iout at output 204 becomes a one variable function of the voltage VDAC at the intermediate voltage point 214 as follows:
Equation 13 lout = G (VDAC , VforCe)
This is illustrated in Figure 9, where the vertical line representing NADC-Nforce have been superimposed on the load lines. As shown in figures 9 and 10, the intersection points uniquely relate the current Iout at output 204 and the voltage VDAC at the intermediate voltage point 214.
With reference to figure 8, by forcing a voltage VADC at output 204 approximating a particular Vforce applied at input 202, the voltage VDAC at the intermediate voltage point 214 will settle at a corresponding N DAC- Suppose the relationship G in equation 13 is known, then the current Iout at output 204 can be deduced by simply substituting NDAC=N DAC into equation 13 to obtain:
Equation 14 lout = G (V DAC . Nforce)
Using the relationship in equation 14, a current measurement mechamsm can be derived as follows. First, it can be seen in figure 8 that the value of the voltage VDAC at the inteπnediate voltage point 214 is reconstructed from the digital value koAC at point 87 via a DAC 210. If we- denote the LSB voltage of the DAC 210 by VLSB-DAC, then mathematically VDAC can be expressed as follows: Equation 15
' DAC ~ "-DAC X LSB-DAC
When the circuit in figure 8 is in equilibrium, VDAC =V DAC- According to equation 15, the value of koAC will be given by: Equation 16
V DAC k ^DAC ~ k DAC
V L,SB-DAC
By substituting equations 7 and 16 into equation 14, the value of the current Iou at output 204 can be deduced as follows: Equation 17 ut = G(v"DAc. Vforce) = G(k*DAc x VISB_DλC,kβιreβ x Vj[SB_ DC)
As VLSB-DAC and VLSB-ADC in equation 17 are constant scale factors, equation 17 can be simplified as follows:
Equation 18 ut = G vAc * V SB_ c,kforce x. VLSB_ΛDC) = G{k*DAc,kforce)
The relationship described in equation 18 can be implemented by the circuit shown in Figure 11.
The system depicted in figure 11 includes the same system as that is depicted in figure 8 in addition to a current measuring circuit. The current measurement circuit includes search logic 90. Search logic 90 receives a signal conveying a forced voltage signal and a signal conveying the target voltage signal at intermediate point 214 (or alternatively at point 87) and processes these signals to derive a measurement of the current flowing at the output 204 of the voltage generating circuit. In the specific example depicted in figure 11, the current measurement circuit includes a first input for receiving the digital representation of the second voltage signal koAC 87, a second input for receiving a digital approximation of the forced voltage signal kFoRCE 84, search logic 90 and an output 92. The search logic 90 derives a certain current measurement on the basis of the digital representation of the second voltage signal kϋAC 87 and the digital approximation of the forced voltage signal kFORCE 84. The certain current measurement is released at the output 92 of the current measurement circuit. When the system is in equilibrium, the certain current measurement is indicative of an approximation of measurement of the current flowing at the output of the voltage generating circuit when the forced voltage is applied to the output 204. It will be appreciated that although the search logic 90 has been described as receiving signals in digital format, search logic 90 may be implemented such that it may receive signals in analog format without detracting from the spirit of the invention.
In accordance with a specific implementation, the search logic of the current measurement circuit includes a data structure in the form of a lookup table 90 containing G . The data structure may be stored on any suitable memory unit such as a RAM, ROM, PROM, EPROM and EEPROM. In a specific non-limiting implementation, the data structure is stored on a RAM device. The values k* DAc 87
and kforce 84 are fed into lookup table 90 to derive Iout. When the system is in equilibrium, the value of Iout can be computed using equation 18.
Calibration Techniques
In a specific implementation, to perform a force-voltage current measurement, a lookup table containing G 90 is used. In this section, two different examples of methods of the calibration procedure for generating the required lookup-table entries for G are described. It will be readily apparent that methods other than those described herein below may be used for generating the lookup table containing G 90 without detracting from the spirit of the invention.
Method 1
In a first specific example, if kAoc is forced to a constant value approximating kforce, equation 18 can be written as follows:
Equation 19
Equation 19 indicate that Iout would become a one- variable function of k DAC- Alternatively, we can write the inverse relationship of equation 19 as follows:
Equation 20
* ,A-i where k DAC is a function of Iout, and G is the inverse function of G. The relationship in equation 20 can be obtained by the circuit shown in figure 12. In the circuit, voltage
NADc at output 204 is held constant at the desired forcing voltage dictated by kforCe 84.
The inverse function G is found by sweeping an external current reference Isweep 94 over a range of current values, followed by recording k DAC 87 at each current step. The values of k DAC may be recorded in any suitable readable memory device. In a
non-limiting implementation, the values of k DAC 87 are recorded on a RAM. In other words, for a given combination of kforCe 84 and Isweep 94, a corresponding value of k DAC i recorded.
Once the inverse mapping G A-i is known, the input and output variables can be interchanged to obtain the function in equation 19. The resultant mapping G can then be used in the current measurement circuit in figure 11. Note that the lookup table G 90 needs to be re-calibrated using this procedure should a different forcing voltage Nforce/ kforce be needed.
Method 2
A drawback of a force-voltage current measurement with calibration method 1 described above is that a large number of calibration points is required before an actual measurement can be done. For example, an n-bit current measurement will require a calibration of 2n points for the function G . If the number of actual current measurements is small, a significant amount of test time will be wasted to generate the unused entries of the lookup table G 90.
This section describes a second specific example of a calibration scheme that can avoid unnecessary calibrations. To achieve this, the current measurement system is calibrated after an actual measurement is made. The details of this algorithm are described below.
A plot of <_r defined in equation 20 is shown in Figure 13. It can be seen that for the same forcing voltage given by a constant k
ADC-kf
orce 84, voltage k
DA
C 87 is a function of I
out at the output 204. Suppose
when the voltage at the external load R
2 206 in figure 8 is forced to approximate N
fo
rce- According to figure 13, the corresponding k
* DAC will be given by k
* DAc
x- Mathematically, this can be expressed as follows:
Equation 21
Now, suppose the output node 204 is connected to a current source at a value I est 96 as shown in figure 14. It can be seen from figure 13 that when Iout is not equal to Ix (e.g., when Iout=Itestι or IouHtest-D, the corresponding k* DAC will not equal k* DACχ- The principles of this calibration algorithm is to use the setup in figure 8 to first measure k* DACx when Iout is equal to the unknown current Ix. Then, a current reference is applied to the system in a manner shown in figure 14. A search can be conducted to find the required Itest value that generates k DAC = k DACX- The resultant Itest value from the search will be the value of the unknown current Ix.
It can be seen from figure 13 that when Itest is set to Itesti^x, the corresponding k DACI is bigger than k* DACx- From this, we can conclude that:
Equation 22 k*DAC > k*DACx when Itest < Iχ
Similarly, by observing the intersection point at Iou ltest-^l , it can be said that
Equation 23 k*DAc < k* DACχ when Itest > Ix
The relationships in equations 22 and 23 provide the basis for the calibration search algorithm. In each iteration, Itest is set to a value and the corresponding k DAC will be compared with k DACX, the comparison result can then be used to increment/decrement Itest in the next iteration. The detail of this algorithm is summarised in Table 1.
Tablel - Calibration Search Algorithm
The calibration algorithm described in Table 1 can be implemented by the calibration circuit 102 shown in figure 15. The calibration circuit 102 includes a current DAC (ID AC) 104, a digital comparator module 108 and a digital integrator 106. In the figure, ID AC 104 is a current DAC whose output current I
test can be controlled digitally. Upon equilibrium, k
DA
C (87) = k
DA
CX (HO), the digital value of Itest 112 would be equal to the digitised value of the unknown current I
x.
The advantage of this calibration algorithm is that for an n-bit current measurement, a calibration of the 2n points for function G is not required. For example, when the search algorithm in Table 1 is implemented using the step search circuit in figure 15, on average only 2n_1 calibration points are required. If the search algorithm in Table 1 is implemented by a binary search algorithm, only n calibration points are required for each measurement.
Force-Current-Measure- Voltage Algorithms
In a second example of implementation of the invention, the input 202 (shown in figures 2a and 2b) is adapted for receiving a forcing parameter signal indicative of a forced current signal Iforce.
The Search Algorithms
The objective of the force-current algorithms is to control the voltage VDAC at intermediate voltage point 214 or voltage VADC at output 204 using the system in figures 2a and 2b such that the current Iout at output 204 will be set to approximate the desired forcing current IforCe applied at input 202.
Search Variables
From equation 6, it can be seen that by varying voltage VDAC intermediate voltage point 214, the value of Iout can be set to approximate a desired value, Iforoe. A search of
the voltage VDAC can be implemented using the system shown in figure 16. The desired VDAC voltage is defined as V DAC- Mathematically, this can be expressed as follows:
Equation 24 Iforce = fl(y DAC)
Figure 17 depicts a graphical view of the search process used to obtain N DAC- For a particular IforCe applied at input 202, the operating VADC (labelled N ADC) corresponds to the intersection of a horizontal line representing Iouι~I_brce and the load line for R2 labelled point B. The search algorithm adjusts the voltage VDAC such that the load line for internal load Ri 216 intersects point B. The corresponding VDAC is then V DAC-
VADC may also be viewed as a search variable. According to equations 5 and 6, we can write:
Equation 25 lout = f2 (fl_1(VADc) ) = f3 (NADC)
Equation 25 indicates that if voltage VADC an output 204 can be controlled, Iout at output 204 can be set to a desired value by searching for the corresponding VADC- A search of voltage VADC at output 204 can be implemented using the system shown in figure 18. Figure 18 shows the same system depicted in figure 2b but with the search unit 208 designed such that voltage VADC at output 204 is set via a feedback network. In this non-limiting implementation, the search unit 208 includes a "VADC Search Logic" circuit 304, a digital comparator module 302 and a digital integrator module 300. The "VADC Search Logic" circuit 304 implements a search algorithm where voltage VADC is a search variable. When the Iout at output 204 is equal to Iforce applied at input 202, the desired voltage VADC value is given by V*ADC in:
Equation 26 Iforce
= f->(N ADC)
Figure 19 provides a graphical view of the search process used to obtain N A
DC- F°
r particular current I
fo
rce applied at input 202, the operating V
DA
C (labelled N
DA
C) corresponds to the intersection of a horizontal line representing
and the load line for R
2, labelled point C in the graph of figure 19. The search algorithm adjusts N
ADC such that the vertical line representing VA
DC intersects point C. The corresponding VAD
C is then N ADC-
Convergence Conditions
From equations 24 and 26, it can be seen that when Iout =Iforce, the following two conditions will be satisfied:
Equation 27
VDAC = V DAC
Equation 28
* VADC = V ADC
Therefore, a search algorithm can determine if Iout-Iforce by observing either VDAC or VADC- This means that either equation 27 or 28 can be used as a convergence condition of the search.
The Four Search Algorithms
From the above discussions, it has been shown that a force-current search algorithm can be implemented by using either VDAC or VADC as the search variable. Also, the convergence condition can be determined by observing either VDAC or VADC- Therefore, there are at least four possible variations for the force-current algorithm, as shown in Table 2.
Table 2 - Four Force-Current Search Algorithms
The descriptions for four force-current algorithms will be presented in the following sections.
Voltage Measurement
For the force-current-measure-voltage operation, the voltage VADC at output 204 must be measured after the force-current algorithm is applied. From the general architecture of the System in figure 2b, it can be seen that the value of the voltage VADC at output 204 can be readily measured by reading the value kADc from the ADC 212 at the end of the force-current searching process.
Convergence Criteria
In the search control logic implemented by search unit 208 of a force-current system, the convergence conditions are observed by evaluating a function (or its inverse) that relates the quantities koAC, kADc and Iout. That function will be described in this section.
Consider the load lines of internal load Ri 216 in figure 20. It can be seen from this figure that voltage VADC at output 204 is dependent on both the voltage VDAC at the intermediate voltage point 214 and current Iout at output 202. Mathematically, this can be expressed as follows:
Equation 29 NADC = H (NDAC , lout)
If current Iout at output 204 is kept constant at a value that approximates Iforce, voltage NADC at output 204 becomes a one variable function of VDAC as follows:
Equation 30
NADC = H (NDAC , Iforce)
This is illustrated in figure 20, where the horizontal line representing IoutHforce is shown in the load lines for internal load Ri 216. As can be seen, the intersection points uniquely relates voltage VADC and voltage VDAC in a manner such as that shown in figure 21.
If we denote the LSB voltage of the DAC 210 used in the system 200 by VLSB-DAC, then VDAC can be represented by a digital number koAC defined in equation 15. Similarly, if the LSB voltage of the ADC 212 is defined as VLSB-ADC, VADC can be represented by a digital value kADc as follows: Equation 31
"ADC = ^ADC X * LSB-ADC
Substituting equation 15 and 31 into equation 30, the following relationship is obtained: Equation 32 ADC X " LSB-ADC = " >ΛC X "LSB-DAC ^ force)
As VLSB-DAC and VLSB-ADC in equation 32 are constant scale factors, equation 32 can be simplified as follows: Equation 33
, _ " pAC X "LSB-DAC > I force) _ fa n_ r
KADC ~ y- ~ H KDAC ' 1 forc )
* LSB-ADC
On the other hand, it can be seen from equation 30 that if current Iout is kept constant at Iforce, voltage VADC i a one variable function of VDAC From this, the reverse relationship can be expressed as:
Equation 34
The relationship in equation 34 is illustrated in figure 22, where the horizontal line representing I0Ut =Iforce is superimposed on top of vertical lines representing different values of VADC- The intersection points are circled in the figure. The intersection point at VADC^VADCI, labelled point D, will be taken as an example. For VADC=VADCI, the corresponding value of VDAC according to equation 34 is given by the load line of internal load Ri 216 that intersects with Point D. Therefore the value of VDAC at point D is given by the following:
Equation 35
' DAC ~ " V ADC\ ' -lout ) ,,, =/ <,„»
If kDAC and kADc in figure 18 are defined by equation 15 and 31, equation 34 becomes: Equation 36
As VLSB-DAC and VLSB-ADC in equation 36 are constant scale factors, equation 36 can be simplified as follows: Equation 37
The functions H and H
_1 defined in equations 33 and 37 are used in the four force- current search algorithms. A summary of the algorithms is presented in Table 3.
Table 3 - Force current search algorithms
Table 4 is an index to the figures corresponding to the Force-Current Search Algorithms described in the specification. It will be appreciated that binary searches can also be performed by replacing the integrators 352 362 372 376 in Figure 23 (a) to • (d) with Successive Approximation Registers (SARs) without detracting from the spirit of the invention.
Table 4 - Force-Current Search Algorithms - Diagrams and Descriptions
The Four Search Implementations
In this section, details of the four search algorithms will be described. Note that because of the similarities in the four algorithms, the reader should get the basic idea from any one of the four algorithm descriptions and may not need to read the other descriptions. In addition, the algorithms are described for signals represented in digital format. It will be appreciated that corresponding algorithms for signals represented in the analog domain may be used without detracting from the spirit of the invention. Such corresponding algorithms and will become apparent to a person skilled in the art in light of the present specification and as such will not be described further here.
Vary-Vr>Ac-Comτ>are-V Anc (Algorithm 1)
The plot in figure 6 can be superimposed with that in figure 20 to form a load line plot in figure 24. It can be seen that out of all the three V
DAC voltages, N
DAC2 will yield Iout=Iforce, i.e.,
Also, at N
DAC2, the corresponding VADC voltage (NADC2) follows the following relationship:
Equation 38
NADC2 = H (NDAC2 , Iforce)
Therefore, it follows that:
Equation 39
VADC = H (VDAC , Iforce) when NDAc = N*DAC
Moreover, it can be seen that for NDACi<NDAC2, the corresponding VADCI is given by:
Equation 40
VADCI > H (VDACI , Iforce) Therefore,
Equation 41 VADC > H ( VDAc , Iforce) when VDAC < V*DAC
Similarly, by observing VDAC3, it can be shown that:
Equation 42
VADC < H (VDAc , Iforce) when VDAC > V*DAC
The relationships in equations 39, 41 and 42 provides the search algorithm required to force I0ut =Iforce- In each iteration, voltage VDAC is set to a value and the corresponding VADC will be compared with H(VDAC , Iforce)- The comparison result can then be used to increment/decrement VDAC in the next iteration. Using the definitions of oAC, kADC and H in equations 15, 31 and 33, the conditions in equations 41 and 42 can be summarised into the force current algorithm in Table 3. The corresponding circuit implementation is shown in Figure 23 a). As shown, the search unit 208 implemented in accordance with this first algorithm includes a lookup table 354 containing the function H , a digital comparator 350 and a digital integrator 352.
Var -V ATicrCompare-V AΏC (Algorithm 2)
An alternative force-current approach can be derived by interpreting the load line plot in figure 24 in another manner. Let voltage VADC be the search variable and the desired VADC that yield I0ut=Iforce be V* ADC- It can be seen from figure 24 that out of all
the three VADC voltages, VADC2 will yield i.e., V*ADC=VADC2- Also, at VADC2 the corresponding VDAC voltage (NDAC2) follows the following relationship:
Equation 43
NADC2 = H (NDAC2 , Iforce)
Therefore, it follows that:
Equation 44
NADC = H (NDAC , Iforce) when NADC = N*ADC
Moreover, by observing the intersection points at VADC = VADCI and VADC = VADC3, it can be shown that:
Equation 45
VADC > H (VDAC , Iforce) when VADC < V*ADC
Equation 46 NADC < H (NDAC , Iforce) when NADC >N* ADC
Using the definitions of koAC, kADC and H in equations 15, 31 and 33, the conditions in equations 45 and 46 can be summarised into the force current algorithm in Table 3. The corresponding circuit implementation is shown in figure 23 (b). As shown, the search unit 208 implemented in accordance with this second algorithm includes a lookup table 354 containing the function H , a first digital comparator 356, a first digital integrator 358, a second digital comparator 360 and a second digital integrator 362.
Vary-V c-Conxpare-VnA (Algorithm 3)
The graph in figure 6 can be superimposed with that in figure 22 to form a load line plot in figure 25. It can be seen that out of all the three VADC voltages, NADC2 will yield i.e., N*ADC=:NADC2. Also, at VADC2 the corresponding VDAC voltage (VDAC2) follows the following relationship:
Equation 47
Therefore, it follows that:
Equation 48
VDAC = _1 (Vmc , Iforce) |/M=/_ when VADC = V*ADC .
Moreover, it can be seen that for VADCI VADC2, the corresponding VDACI is given by:
Equation 49
Equation 50 VDAC < H_1()^c,//oree) \Im _Ifirce when VADC < V*ADC
Similarly, by observing NADC3, it can be. shown that:
Equation 51 V
DAC >
_
Iforce when VADC > V
*ADC
The relationships in equations 48, 50 and 51 provides the search algorithm required to force I
ou
t-I
force- In each iteration, VA
DC is set to a value and the corresponding VDAC will be compared with
»
me comparison result can then be used to increment/decrement VA
DC in the next iteration. Using the definitions of kϋAc, k
ADC and H
1 in equations 15, 31 and 37, the conditions in equations 50 and 51 can be summarised into the force current algorithm in Table. 3. The corresponding circuit implementation is shown in Figure 23 c. As shown, the search unit 208 implemented in accordance with this third algorithm includes lookup table 364 containing the
function H
"*, a first digital comparator 366, a first digital integrator 368, a second digital comparator 370 and a second digital integrator 372. Varv-VnAc-Comvare-Vmr, (Algorithm 4)
An alternative force-current approach can be derived by interpreting the load line plot in figure 25 in another manner. Let VDAC be the search variable and the desired VDAC that yield I0ut=Iforce be V*DAC It can be seen from figure 25 that out of all the three VDAC voltages, VDAC2 will yield Iout=Ifbrce. i-e., V*DAC=VDAC2- Also, at VDAC2, the corresponding VADC voltage (NADC2) follows the following relationship:
Equation 52
Therefore, it follows that: Equation 53
VDAC = H~ ΦADC' I force) cm,=-/' f,_or, when NDAC = N DAC
Moreover, by observing the intersection points at VDAC = NDAci and VDAC = VDAC3, it can be shown that:
Equation 54
Equation 55 VDAC > H-'iV^cI^ \Im _Ifom when VDAc > V* DAC
Using the definitions of koAC, kADC and H_1 in equations 15, 31 and 37, the conditions in equations 54 and 55 can be summarized into the force current algorithm in Table 3. The algorithm described in Table 3 can be implemented by the circuit
shown in figure 23 (d). As shown, the search unit 208 implemented in accordance with this fourth algorithm includes lookup table 364 containing the function H " , a digital comparator 374 and a digital integrator 376. It can be observed that the circuit in figure 23 (d) including 374, 376 and the feedback line from 376 to 374 is a unity gain buffer. As such, the circuit can be simplified to the implementation shown in figure 26.
In the circuit in figure 26 the search unit 208 is implemented by a memory module 364, such as a RAM. In a non-limiting implementation, the search unit 208 is implemented by a RAM without any addition digital circuitry for performing the searching functionality.
Calibration Techniques
In the first and second force-current algorithms described above, a lookup table 354 containing the function H is used in the circuit implementation as shown in figures 23a) and 23b). Also in the third and fourth force-current algorithms, a lookup table
364 containing the function H -1 is used in the circuit implementation as shown in figures 23c) and 23d). The calibration procedures for these functions will be described herein below.
The function H defined in equation 34 can be found using the circuit shown in figure 27. In the circuit, Iout at output 204 is held constant at the desired forcing current IforCe by current source 380. The function H is found by sweeping koAC over the full-scale range of the DAC 210, followed by recording kADC 85 at each step.
On the other hand, the function H , defined in equation 37, can be found using the circuit shown in figure 28. In this circuit, Iout at output 204 is held constant at the desired forcing current Iforce by current source 380. The function H"1 is found by sweeping kADC over the full-scale range of the ADC 212, followed by recording koAC 87 at each step.
Note that, alternatively, the function H can be obtained by first finding the functional relationship H -1 followed by a switch of input and output variables. Similarly, the function H"1 can be obtained by switching the input and output variable after finding H .
The lookup table H (or H_1, whichever is employed in the force-current search algorithm) needs to be re-calibrated should a different forcing current IforCe be needed.
Algorithm Modifications for an Inverting Load
The section below describes the circuit modifications when the internal load Ri 216 used in the system 200 is an inverting load.
Force-Voltage-Measure-Current Algorithms
If the internal load Ri 216 of the system 200 follows the inverting property defined in equation 3, the feedback loop in Figure 8 needs to maintain negative feedback by switching the polarity of the comparator 82. The modified circuit is shown in figure 29 where the comparator 82 of figure 8 has been replaced by comparator 400 in figure 29. Note that the same change also applies to the measurement and calibration circuits in figures 11, 12 and 14.
The current measurement and the first calibration method described above for a non- inverting internal load Ri 216 do not need to be modified for an inverting Ri because V DAC (as well as k*DAc) is still a one-to-one corresponding function of Iout for any particular VADC =Vforce- However, the second calibration method described needs to be modified for an inverting Ri. A modified calibration search algorithm is shown in Table 5. The modified calibration search circuit in figure 15 is shown in figure 30 where the comparators 82 and 108 of figure 15 have been replaced by comparators 400 and 402 respectively in figure 30.
Table 5 - Modified Calibration Search Algorithm for an Inverting Ri
Force-Current-Measure-Voltage Algorithms
When the internal load Ri 216 of the system 200 follows the inverting property as defined in equation 3, the feedback loop in the general force- VADC architecture in figure 18 needs to maintain negative feedback by switching the polarity of the comparator. The modified circuit is shown in figure 31 where the comparator 302 of figure 18 has been replaced by comparator 600 in figure 31. Similarly the VADC search logic module 304 in figure 18 is replaced by modified VADC search logic module 604 for certain algorithms (as in figure 32).
The modifications for each force-current algorithm is described herein below.
Varv-VnAc-Comyare-V oc (Algorithm 1)
When an inverting internal load Ri 216 is used, this search algorithm is modified in order to maintain convergence of the target current value Iforce- The modified search algorithm and its implementation are shown in Table 6 and figure 32. As shown, the search unit 208 implemented in accordance with this first algorithm when an inverting internal load Ri 216 is used includes a lookup table 364 containing the function H Λ, a digital comparator 600 and a digital integrator 602.
Table 6 - Force current algorithm for an inverting Ri (vary- VDAc-compare- VADC)
Vary-V AncyCompare-V mc (Algorithm 2)
There is no change in this search algorithm when an inverting internal load Ri 216 is used. However, because this algorithm has a force- VADC circuit implementation, the modification in figure 31 is required. The modified circuit is shown in figure 33. As shown, the search unit 208 implemented in accordance with this second algorithm includes a lookup table 614 containing the function H , a first digital comparator 612, a first digital integrator 610, a second digital comparator 608 and a second digital integrator 606.
Vary-VAnc-Compare-VnAc. (Algorithm 3)
When an inverting internal load Ri 216 is used, this search algorithm has to change in order to maintain convergence of the target current value. The modified search algorithm is shown in Table 7. Also, because this algorithm has a force- VADC circuit implementation, the modification in Figure 31 is required. The modified circuit is shown in figure 34. As shown, the search unit 208 implemented in accordance with this third algorithm includes a lookup table 616 containing the function H , a first digital comparator 612, a first digital integrator 610, a second digital comparator 608 and a second digital integrator 606.
Table 7 - Force current algorithm for an inverting Ri (vary- VADC-compare- VDAC)
Vary-VnAc-Compare-VnAC (Algorithm 4)
There is no change in this algorithm or its implementation if an inverting Ri is used.
5 Special case; Using an Internal Linear Resistive Load Ri 216
This section deals with the special case where the internal load Ri 216 used in the system 200 is a linear resistor. The general architectures of the system that forces VDAC and VADC are shown in figure 35 and figure 36. In both figures, the internal load 0 Ri 216 is a linear resistor and is denoted by RIL. In figure 36, the search unit 208 includes search logic 704, a digital comparator 702 and a digital integrator 700. Upon equilibrium, the current Iout at output 204 in both circuits is given by:
Equation 56 ς γ __ ^ADC X ' LSB-ADC ~ ^DAC X ' LSB-DAC , T J 1 out ~ r> ~r x offset
K\L
where I0ffSet is a current offset term resulted from the offset voltages of the DAC 210 and the ADC 212. A calibration process is required to determine the values of the constants RiL and I0ffset. After these values are found, the lookup tables can be readily 0 constructed for the force-voltage-measure-current or the force-current-measure- current algorithms.
From equation 56, it can be seen that the quantities Iout, kADc and koAC are linearly related. As we will see in the following sections, this linearity property will 5 dramatically deduce the amount calibration time required.
Current Measurement
Consider the force- voltage-measure-current circuit in figure 37. It is the same as the circuit in figure 11 except that internal load Ri 216 is a linear resistor denoted by RIL-
For this circuit, we can write G defined in equation 18 as:
Equation 57
T - il-* v \ - ^ADC x VLSB-ADC ~ k DΛC X VLSB-DAC , r
1out - DAC, Vforce) + 1 offset
K\L
where the values of RIL and I0ffset can be found using the calibration method described further on in the specification.
The relationship in equation 57 indicates that the lookup table G 706 in the current measurement system (shown in figure 37) can be generated byttwo calibration points.
Moreover, from equation 57, it can also be seen that the lookup table G 706 need not be re-calibrated if a different forcing voltage is required. This is different from the lookup table G 90 shown in figure 11 for the generalized current measurement system, where the function G needs to be re-calibrated whenever a different forcing voltage Nforce) is needed.
Current Generation
With the relationship in equation 56, we can write H defined in equation 33 as follows:
Equation 58 π(l T \ - lr - - f°rce ~Ioffset R\L ^ Jr „ VLSB-DAC n KDAC ' J force ) ~ KADC ~ γ V KDAC Λ 9}
' LSB-ADC ' LSB-ADC
Similarly, we can write H J defined in equation 37 as follows:
Equation 59
The relationships in equations 58 and 59 mean that the lookup table H and H-1 in the force-current system described previously for the case of a general internal load Ri 216 can be generated by two calibration points when internal load Ri 216 is a linear resistor. Moreover, from equations 58 and 59, it can also be seen that the lookup tables need not be re-calibrated if a different forcing current is required. This is different from the lookup tables for the generalised current forcing system in Section 2.4, where the functions H and H"1 need to be re-calibrated whenever a different forcing current (Iforce) is needed.
Calibration Circuits
The force- voltage circuit in figure 38 is analogous to the front end of the system in figure 35. In figure 38, a reference current source 708 with value Iref is connected to the resistor 216 at output 204. The voltage VDAC at output 204 is forced by the digital value koAC-cai 709. When the circuit in figure 38 is in equilibrium, the value of voltage VADC at output 204 is represented by kADC-cai 711. The reference current source Iref 708 can be expressed as:
Equation 60
T _ ^ADC-cal X ' LSB-ADC ~ ^DAC-cal X ' LSB-DAC , γ 1ref - Ό "TJ offset
K L
Another force- voltage circuit is shown in figure 39. This circuit is similar to the force- VADC architecture shown in figure 36. The voltage VADC at output 204 is set by the digital value kADC-cai 713. When the circuit is shown in figure 39 is in equilibrium, the value of voltage VDAC at intermediate voltage point 214 is represented by koAc-cai 714. The expression in equation 60 can also be used to describe current Iref 708.
A purpose of the calibration process is to determine the values of the constants R^ and loffset- To find these two constants, two calibration points are used. Note that either one of the circuits in figure 38 and figure 39 can be used as the calibration circuit as they both follow equation 60.
For the circuit in figure 38, a value of koAC-cai 709 and a reference current Iref 708 is set at each calibration point. Then the resultant kADC value, kADC-cai 711, is recorded. For the circuit in Figure 39, a value of kADC-cai 713 and a reference current Iref 708 is set at each calibration point. Then the resultant koAC value, koAC-cai 714, is recorded.
The values of kADC-cai or the two calibration points will be designated as kADC-caii and kADC-cai2- Similarly, the calibration values for koAC-cai and Iref can be written as kDAC_caiι, kDAC-cai , Lefi and Iref2- Using equation 60, two equations can be generated after two calibration points: Equation 61
T _ k ADC-call X ' LSB-ADC ~ ^DAC-call X ' LSB-DAC , j Xref\ - Ώ ~r l offset
R\L
Equation 62
T _ ^ADC-call X ' LSB-ADC ~ 'ZDAC-cal2 X ' LSB-DAC , r
Xref2 - Ώ "^ offset
K\L
Equations 61 and 62 can be solved to yield the values of RIL and loffset as follows:
Equation 64
*
■ offset
Special Case: Simplified Calibration and Measurement
Equations 57, 58 and 59 can be used to generate a lookup table with the generic calibration procedure described previously in the specification. The section below shows how to simplify equations 63 and 64 to reduce the computational complexity of generating a lookup table for a linear resistive load.
Equations 57, 58 and 59 can be simplified if constraints are imposed on the design of the system and on the calibration procedures. This section describes an example of such a set of constraints that enables effective implementation of the system with a linear element. It will be readily appreciated that other methods for simplifying the computations may also be used without detracting from the spirit of the invention.
To avoid unnecessary arithmetic due to LSB conversions, the system 200 can be designed such that:
In other words, the LSB voltage of ADC 212 and DAC 210 are the same. In the calibration procedure, the force- VADC circuit in figure 39 is used. The two calibration points are given in Table 8.
Table 8 - Calibration Points
The zero reference current I
ref2 can be easily set by merely disconnecting the output 204. Hence, only one calibration point Ifo
rce-ref requires an external reference. The calibration process is summarised in Table 9 with reference to figure 39.
Table 9 - Calibration of the System with Linear Ri 216
With the system requirement in equation 65 and the calibration points in Table 8, equations 57, 58 and 59 can be simplified as follows:
Equation 66 k DAC kDAC_cal2 + kβrce_ref kforce
Iout =
G(
k*DAC , V
force " force— ref
Equation 67 ADC ~
+ EC* force)
where:
Equation 69
7
~/ force -* force) ' ~
r + V
1 force-ref
force-ref
These formulas can be reasonably easily implemented by a digital circuit. The force- voltage-measure-current process using the calibrated values is summarised in Table 10 with reference to figure 37.
TablelO - Force- Voltage Current Measurement Procedure
The calibrated system can also be used to force any arbitrary current Iout at output 204 to the external load R2 206 and measure the voltage VADC at output 204. Any force- current algorithm in figures 23 or 26 can be used. This process is summarised in Table 11.
Table 11 - Force-Current Voltage Measurement Procedure
CIRCUIT IMPLEMENTATIONS
The following part of this specification describes specific examples of implementations of the general system 200 shown in figures 2a and 2b. Circuit implementations of the force- voltage-measure-current and the force-current-measure- voltage algorithms will be described.
General Architectures of the Force-Voltage/Force-Current Algorithms
In a non-limiting implementation, the system 200 shown in figure 2b may be implemented using digital logic components and a front-end circuit. The details of the front-end circuit are provided herein below.
Force- Voltage-Measure-Current Algorithm
The general structure of the system for implemented a force- voltage-measure-current circuit is shown in figure 40. As depicted, the system includes a lookup table 706 containing G and a front-end circuit, referred to as 'NADC-Forcing circuit" 721 in figure 40. In this non-limiting implementation, lookup table 706 is implemented by a RAM. Lookup table 706 releases a current measurement at output 715. It will be appreciated that other suitable memory devices may be used without detracting from the spirit of the invention. The VADC-Forcing circuit 721 is shown in isolation in figure 41. Details of the VADC-Forcing circuit 721 will be described further on in the specification.
Force-Current-Measure- Voltage Algorithm
With reference to figures 23 a), b), c), d) and figure 26, two basic architectures have been described for the Force-Current-Measure- Voltage Circuit, depending on whether the search variable is the voltage VADC at output 204 or voltage VDAC at the intermediate voltage point 214. The two basic architectures are shown in figures 42 and 43. As shown, for both of these architectures, the search logic 720 724 accesses a lookup table 722 where either function H or function H_1 is stored. In this non- limiting implementation, lookup table 722 is implemented by a RAM device. It will be appreciated that other suitable memory devices may be used without detracting from the spirit of the invention.
For the force-current architecture that uses voltage VADC at output 204 as a search variable, shown in figure 42, the VADC-Forcing circuit 721 shown in figure 41 is used as a front-end circuit. For the force-current architecture that uses voltage VDAC at the intermediate voltage point 214 as a search variable, shown in figure 43, the value voltage VDAC at point 214 is reconstructed from value koAC at node 87 using DAC 210. The VDAC-Forcing circuit 725 shown in figure 43 and in figure 44 includes DAC 210, internal load Ri 216 and the ADC 212. Details of the VDAc-Forcing circuit 725 will be described herein below.
VDAC-F or cing Circuit 725
In a non-limiting implementation, the architecture of the Vr-Ac-Forcing Circuit 725 in figure 44 includes of a DAC 210 with essentially zero output impedance, hereinafter referred to as a "low-impedance DAC" 210, an internal load element Ri 216 and an ADC 212 with infinite input impedance, hereinafter referred to as a "high-impedance ADC". It is to be appreciated that the expression "infinite input impedance" is meant to designate a device, which in its effective operating range will draw a quantity of current that is considered to be essentially negligible. The components of the VDAC- Forcing Circuit 725 are described in the following sections.
DAC 210 and the Internal Load Element Ri 216
The partial front-end circuit with the low-impedance DAC 210 and internal load Ri 216 is shown in figure 45. The combination of the DAC 210 and internal load Ri 216 as separate components is one example of implementation of a circuit module having digital-to-analog conversion functionality and load functionality. The voltage VADC at output 204 is given by a function of voltage VDAC at intermediate voltage point 214' and current Iout at output 204 as follows:
Equation 70 V^^H^V^,!^)
where the function Hiu is dependent on the internal load Ri 216. The voltage VDAC at intermediate voltage point 214 is described by:
Equation 71
' DAC — ^DAC X ' LSB-DAC
In this description, five alternative specific implementations of the circuit module having digital-to-analog conversion functionality and load functionality in figure 45 will be described. It will be readily appreciated that other configurations of the circuit in figure 45 may be used without detracting from the spirit of the invention.
Two of these implementations are shown in figures 46a) and 46b). The implementations in figures 46a) and 46b) are hereinafter referred to as Load Configurations A and B respectively. In both circuits, a PDM generator 801 generates digital pulses with a specific density of l's (hereinafter referred to as "pulse density") controlled by kDAC When the digital pulses pass through the low-pass filter (LPF) formed by RAX and CAX (where x = 1 to m), the output voltage VADC at output 204 is settled at a specific DC value that is dependent on the pulse density. Also, it can be seen that for a particular pulse density, the DC value of voltage VADC at output 204 will change depending on current Iout at output 204. Therefore, the circuits in figures 46a) and 46b) can be represented by a low-impedance DAC connected with a resistor. The parameters of the equivalent DAC 210 and the internal load Ri 216 shown in figure 45 can be derived as follows. For the circuits in figures 46a) and 46b), if the PDM generator 801 can generate 2n distinct pulse densities, VLSB-DAC of the equivalent DAC 210 (in figure 45) is given by:
Equation 72
where V
DD is the voltage representing the high value of the PDM generator 801. Also, the equivalent internal load Ri 216 is linear and is given by the values in Table 12 below.
Table 12 - Equivalent load resistances of Load Configurations A and B
Alternatively, the circuit in figure 45 can be implemented using other configurations. Consider the three implementations shown in figures 47 (a) to (c) (hereinafter referred to as Load Configurations C to E). In these circuits, the PDM generator 801 and the low-pass filter (LPF) 805 corresponds to the low-impedance DAC 210 in figure 45. If the PDM generator 801 can generate 2n distinct pulse densities, VLSB-DAC of the equivalent DAC 210 in figure 45 is given by equation 72.
Each of the MOS circuits 807 in figures 47 (a) to (c) correspond to the internal load Ri 216 in figure 45. It can be seen that the equivalent internal load Ri 216 for any one of the MOS circuits shown is inverting because the equivalent voltage VDAC at intermediate voltage point 214 (not shown) increases while voltage VADC at output 204 decreases for a particular current Iout at output 204. Note that in practice, the current sources in figures 47(b) and (c) of the MOS circuits 807 will generally have a positive differential output resistance. Hence, I ias is dependent on voltage VADC at output 204. Moreover, load configurations D and E (shown in figures 47(b) and (c)) can also be implemented with the bias current sources removed (i.e., IDias =0). In this case, configuration D (in figures 47(b)) can be used to measure/generate negative
output currents (where lout<0) only while configuration E (in figures 47(c)) can be used to measure/generate positive output currents (where lout>0) only.
The value of voltage VADC at output 204 for the five configurations described above can be easily deduced and is shown in Table 13. In Table 13, voltage VDAC at intermediate node 214 is the equivalent DAC voltage given by equation 71. The functions HNOT, HPMOS and HNMOS are DC transfer functions of the CMOS inverter, PMOS and NMOS circuits shown in figures 48 (a), (b) and (c), respectively, which can be written as:
Equation 75
Vouti = HNOT (Vini , Iouti)
Equation 76
Vout2 = HpMOS (Vjn2 , Iout2)
Equation 77
V0ut3 = HNMQS (Nin3 , Iout3)
It will be appreciated that although the examples shown in figures 46(a), 46(b) and 47(a) to (c) include the PDM generator 801, the latter may be replaced by a suitable general-purpose pulse generator that provides a digital pulse.
ADC 212 (Digital Integration / Successive Approximation)
In a non-limiting implementation, the ADC 212 shown in figure 44 can be implemented by a delta-modulator of the type described in U.S. Patent No. 2,605,361 issued to Cassius C. Cutler et al. in July 29, 1952. The content of this document is hereby incorporated by reference. This implementation is shown in figure 49.
As shown, the analog-to-digital converter module 212 includes an analog comparator 504, a digital integrator 502 and a feedback circuit. The analog comparator 504 receives a signal indicative of the voltage at the output 204 and a tracking voltage Vtrac and generates a difference signal on the basis of the signals received. The digital integrator 502 receives the difference signal and generates successive digital approximations of the voltage signal at the output of the circuit device. The feedback circuit processes the successive digital approximations of the voltage signal to generate the tracking voltage Vtrack and provide the latter to the analog comparator 504. In a non-limiting implementation, the feedback circuit includes a digital-to- analog converter module 500.
In a non-limiting implementation, the DAC 500 in figure 49 can be implemented using the voltage reference described in M. M. Hafed, S. Laberge, G. W. Roberts, "A Robust Deep Submicron Programmable DC Voltage Generator", Proc. IEEE International Symposium on Circuits and Systems, Vol. 4, pp. 5-8, May 2000 and depicted in figure 50. The content of this document is hereby incorporated by reference.
The output of the digital integrator 502, kADC, increases or decreases by a constant amount depending on the result from the comparator 504. Upon equilibrium, the tracking voltage Vtrack released by the DAC 500 will equal voltage VADC- The value k DC will become a digital representation of VADC
In accordance with an alternative specific example of implementation, the ADC 212 shown in figure 44 can be implemented by successive-approximation circuit of the type described in D. A. Johns, K. Martin, Analog Integrated Circuit Design, John Wiley & Sons, Inc., pp. 492-493, 1997. The content of this document is hereby incorporated by reference. As shown in figure 51, the digital integrator 502 in the analog-to-digital converter module in figure 49 is replaced by a successive- approximation register (SAR) 506 module. In this implementation, the Successive- Approximation Register Logic (SAR Logic) 506 binary searches for the value of kADC that equalises VADC and Vrack- The output value kADC will be a quantized representation of VADC given by:
Equation 83
7, _ V ' A.D: C
*ΆDC ~ V L,SB-ADC
where VLSB-ADC is the LSB voltage of the ADC circuit 212. If we denote the LSB voltage of the DAC 500 that is used in the ADC circuit in figures 49 and 51 by VLSB- DAC(ADC), the value of VLSB-ADC will be given by:
Equation 84
' LSB-ADC = ' LSB-DAC(ADC)
ADC 212 (Delta Modulator)
In yet another alternative implementation, the ADC 212 is implemented by a delta- modulator of the type described in D. J. G. Janssen, "Delta Modulation in DVM Design", IEEE Journal of Solid-State Circuits, Vol. SC-7, pp. 503-507, Dec. 1972. The content of this document is hereby incorporated by reference. A non-limiting implementation of the circuit is shown in figure 52. As shown, the ADC 212 includes an analog comparator 504, a D-Flip-Flop (D-FF) 508, an RC filter 512 and a frame counter 510. It will be appreciated that although the example shown in figure 52 includes a the frame counter 510, the latter may be replaced by a general purpose digital filter implementing accumulation functionality such as a digital integrator for example.
At equilibrium, the tracking voltage Vtrack will equal voltage VADC at output 204. The DC value of Vtrack (and thus VADC) can be deduced by observing the density of l's (hereinafter referred to as "pulse density") from the D-FF 508. The frame counter 510 captures a frame of bits from the output of the D-FF 508 and counts the number of l's in a frame. The resultant count kADc will become a digital representation of voltage VADC- For example, if the length of the frame captured is 2n bits, the output value kADc will be a quantized representation of VADC given by equation 83, where:
Equation 85
Functional Relationship Between kAnc, k andln t
When the VoAc-Forcing circuit 725 (figure 44) is used to implement the first and second current-searching algorithm, the corresponding function H defined in equation 33 is dependent on the internal load Ri. From equations 70, 71 and 83, H can be written as:
Equation 86
where VLSB-ADC is the LSB voltage of the ADC 212 in the VoAC-Forcing circuit 725. Using equation 86, the relationships in Table 13 can be rewritten as functions H listed in Table 14.
Table 14 - Value of kADC for different load configurations
Load Definition of H Configuration
A Equation 87
V L,SB-ADC
B Equation 88
^DAC X " LSB-DAC " " RB +∑RAI R.
;=1
H(fcDAC>Io, ) = - F L,SB-APC
Equation 89 ^NOT KpAC X ' LSB-PAC ^O.
H(kpAc>Iout) - r L,SB-ADC
D Equation 90
H(kDAC,I0Ut) tt PMOS DAC X " LSB-DAC ' ^OΗΓ )
V L,SB-ADC
E Equation 91
V
ADC-Forcing Circuits 721
VADC-Forcing Circuits 721, of the type shown in figure 42, are used in the force- voltage-measure-current algorithm as well as the third and fourth force-current- 5 measure- voltage algorithms. In this section, two specific non-limiting implementations of the VADC-Forcing circuits 721 will be described. It will be readily apparent that other suitable implementations are possible without detracting from the spirit of the invention. Before the description of the two implementations, the definition of a function that relates the key variables of the VAoc-Forcing Circuit 721 10 is provided in the following.
When a VADC-Forcing Circuit 721 is in equilibrium, the quantities koAC, kADC and Iout will be related by a function which is dependent on the internal load Ri 216. That function will be described in the following. Let us define a DC transfer characteristic 15 WRI for internal load Ri 216 such that voltage VDAC at intermediate voltage point 214 is a function of the voltage VADC and the current Iout at the output 204. Mathematically, this can be expressed as follows:
Equation 92
20 VDAC=WR1(NADC, lout)
When the circuit in figure 44 is in equilibrium, koAC can be expressed as:
Equation 93
■J Γ ;L _ ^ j.1 ^ADC x VLSB-ADC - ut ) J KDAC ~~
' LSB-DAC
where VLSB-ADC and VLSB-DAC are the LSB voltages of the equivalent ADC 212 and the DAC 210 employed in the VADC-forcing circuit 721, respectively. Function WRl can be defined as: 30
Equation 94
VfT (lr T \ — Jr - " R\ ^ADC X ' LSB-ADC ■ hut ) πR\ lζADC > I out ) - KDAC 91
' LSB-DAC
For each variation of the VADC-Forcing Circuit 721 described in the following sub- sections, the function WRl will be derived and listed. At the end of the section, we will show that the function WRl for the VAoc-Forcing circuits described is related to the functions G and H used the force-voltage / force-current algorithms.
VAnr-Forcing Circuit 721 (Digital Integration / Successive Approximation)
The VADC-Forcing Circuit 721 in figure 41 is redrawn in figure 53. It can be seen that the VADC-Forcing circuit 721 includes a VoAC-Forcing circuit 725 with a digital comparator 702 and a digital integrator 700. Therefore, a VAoc-Forcing Circuit 721 can be implemented using a VoAC-Forcing circuit 725 described above in addition to the digital logic shown in figure 53.
In a non-limiting implementation where the combination of DAC 210 and internal load Ri 216 is of the type shown in figures 46 a) or b) (type A or B), the equivalent internal load Ri 216 is linear and the corresponding values are shown in Table 12 above.
If an alternative non-limiting implementation where the combination of DAC 210 and the internal load Ri 216 is of the type shown in figures 47a), b) or c) (type C or D or E), the equivalent internal load Ri 216 will be an inverting load. For these implementations, the polarity of the digital comparator 702 in figure 53 should be inverted to maintain negative feedback. This modification is shown in figure 54 where digital comparator 702 was replaced by digital comparator 730.
Similarly to the ADC circuit 212 described above with reference to figures 49 and 51, the digital integrator 700 in figures 53 and 54 can be replaced by an SAR Logic unit such that the value of koAC can be obtained by a binary search.
Upon equilibrium, the function WRX for each load configuration is given by Table 15. In Table 15, VLSB-ADC and V SB-DAC(PDM) are the LSB voltages of the equivalent ADC 212 and the DAC 210. The functions WNOT, WPMOS and WNMOS represent the DC transfer characteristics of the CMOS inverter, PMOS and NMOS circuits shown in figure 48 (a), (b) and (c), which can be written as: Equation 95
Vjni = WNOT (Vouti , louti)
Equation 96
Vjiβ = WpMOS (Vout2 , lout-)
Equation 97 Vjn3 = WNMOS (V0Ut3 , Io t3)
Table 15 - The Value of koAC for different load configurations
Load Definition of the function W R.\ Configuration
A Equation 98
B Equation 99
VAnc-Forcing Circuit 721 (Delta Modulation)
The VADC-Forcing Circuits 721 in figures 53 and 54 can be simplified into a group of circuits derived from a Delta-Modulator structure. Specific examples of implementation of such circuits are shown in figures 55 (a) to (e). Each circuit shown in figure 55 includes a DAC 904, an analog comparator 902 (916 in figures 55 (c) to (e)), a D-FF 906 and a digital filter in the form of a frame counter 900. It will be appreciated that although the example shown in figures 55 (a) to (e) includes frame counter 900, the latter may be replaced by a general purpose digital filter implementing accumulation functionality such as a digital integrator for example. The corresponding set-ups for load configurations shown in figures 46 (a) and (b) and in figures 47 (a) to (c) (i.e. Load Configurations A to E) are shown in figure 55 (a) to (e) respectively.
Upon equilibrium, the voltage VADC at output 204 will be equal to the DC value set by kADC and the DAC 904. As previously described, the value of koAC can be deduced by observing the pulse density from the D-Flip Flop 906 using a frame counter 900.
The function WRl for the different load configurations are given by equations 105 to 109 shown in Table 16. In the table, VLSB-ADC is given by:
Equation 103
VLSB-ADC= VLSB-DAC (Force-vADC)
where VLSB-DAC(Force-vADC) is the LSB voltage of the DAC used in the Force- VADC circuits in figures 55 (a) to (d).
The value of VLSB-DAC in Table 16 depends on the length of a captured frame in the frame counter 900. For a captured frame with 2n bits in length, VLSB-DAC is given by:
Equation 104
V ' DnP.
V L,SB-DAC 2"
Table 16 - Value of koAC for different load configurations
Load Definition of the function W R,\ Configuration
Equation 105
wRl(k _ '\A ADnrCA ' L rS<mB_-DnA_rC K M-Aλl X ' -tΛ,
ADC ' ■'■out)
V L,SB-ADC
B
Equation 106
Equation 107
D Equation 108
XV Or T \ — PMOS ^ADC X " LSB-DAC ' 1 out ) WR\ y^ADC > l out ) Tr
V L,SB-ADC
Equation 109
Comparing the equations in Table 16 to that in Table 15, it can be seen that the structures in figure 55 (a) to (d) can also be mapped into the generalised architecture of the V
ADC-forcing circuit in figure 41. For the load configuration of type shown in figures 46 (a) and (b) (type A or B), the equivalent internal load Ri 216 is linear and the corresponding values are shown in Table 17. For load configurations shown in figures 47 (a) to (c) (type C, D or E) the equivalent internal load Ri 216 will be an inverting load.
Table 17 - Equivalent load resistances
When the VADC-Forcing circuit 721 (figure 41) is used to implement the force- voltage-measure-current or the third or fourth current-searching algorithm, the corresponding functions G and H , defined in equations 18 and 33, will be dependent on the internal load Ri.
Comparing equation 94 to equations 20 and 37, it can be seen that WRl represents G 1 in the force- voltage-measure-current algorithm and H~ in the force-current measurement voltage algorithm, i.e.:
Equation il2
Therefore, for all the VADC-Forcing circuit 721 with WR1 defined in equations 105 to 109, the corresponding G and H for the force- voltage-measure-current algorithm and the third and fourth force-current-measure-voltage algorithm are given by equations 112 and 113.
SPECIFIC EXAMPLES OF IMPLEMENTATION
Example 1 -System with Non-linear Inverting Load Element R7 216
In a first alternative specific example, depicted in figure 56, the system includes a VADC-Forcing circuit described in figure 55 (c) with the DAC 904 implemented by the circuit shown figure 50. A first-order RC filter is used for the DAC 904. In the figure, both the PDM generator and the frame counter 900 have 8-bit (256 distinct levels) resolution. The analog full-scale voltage range is from 0 V to 5 V.
Current Measurement
Using the VADC-Forcing circuit in figure 56, a force-voltage-measure-current system can be constructed as described previously in the specification. The resultant system is shown in figure 57. In this example, let us assume that we need to force 3.75 V at the output 204 (VADC)- According to equations 72 and 103, the value of kADC should be set as follows:
Equation 114
Jr - l-
KADC ~
K force
To calibrate this ammeter for a 5-bit current resolution, 32 currents, each with an increment of 1/32 of the full-scale current, is applied to the output 204 in the manner shown in figure 58. For each current increment, the corresponding k DA
C from the frame counter is recorded. The result is tabulated as that listed in Table 18. In this example, the full-scale current range is from -1 mA to 0.9375 mA. Note that Table 18 is a mapping of the function G
~l defined in equation 18. To find G , we simply invert the second and third column of the table.
Table 18 - A 5 -bit look-up table for the ammeter
After the calibration, the system in figure 57 can then be used to set 3.75 V at an unknown external load 206, from which the appropriate load current Iout flows. The resultant bit code k DAC is then compared to the counts found in Table 18. For example, if an unknown current is applied and a k DAC of 108 is produced, the unknown current will be found to be 0.0625 mA.
Current Generation
Using the VAoc-Forcing circuit in figure 56, the force-current-measure-voltage system shown in figure 33 can be constructed. The resultant system is shown in figure 59. This system employs the third current-forcing algorithm described previously, where
kADC is controlled by a search unit 928 that observes the value of koAC As shown in figure 59, the digital logic 928 includes a look-up table, a digital comparator 930 and an integrator 932.
The lookup table 933 containing H needs to be calibrated whenever a new value of Iforce applied at input 202 is required. That can be done using the set-up shown in figure 60. During calibration, the Iout is set to Iforce by an external current reference 926 while kADC is swept. For each kADC increment, the corresponding value koAC from the frame counter 900 is recorded. The resultant lookup table lists the value of koAC as a function of kADC, i-e-, the function H"1 defined in equation 37. An example lookup- table is shown in Table 19.
Table 19 - An 8-bit look-up table for the current source
After calibration, an external load 206 can be applied to the output 204 of the system as shown in figure 59. A current of value Iforce will be forced into the external load R2 206 when the system is in equilibrium.
Example 2 - System with Linear Load Element R7
In a second alternative specific example, depicted in figure 61, the system is constructed using the VADC-Forcing circuit described in figure 55 (a) with the DAC 904 implemented by the circuit in Figure 50. Again, a first-order RC filter is used for
the DAC 904. In the figure, both the PDM generator and the frame counter 900 have 10-bit (1024 distinct levels) resolution. The analog full-scale voltage range is from 0 V to 2.56 V.
The architectures/operations of the Force- voltage and Force-current systems derived from the VADC-Forcing circuit in figure 61 is similar to those described with regards to the first alternative specific example described above. However, because the internal load element Ri is linear, calibration/measurement method described in connection with a linear resistive internal load Ri 216 can be used.
As both the equivalent ADC and DAC 904 in the VADC-Forcing circuit shown in figure 61 have 10-bit resolution over a full scale of 0 V to 2.56 V, the LSB voltage (VLSB) can be defined as follows:
Equation 115
2.56V
' LSB ~ ' LSB-DAC ~ ' LSB-ADC = 2.5mV 1024
In a non-limiting implementation, the resistance R2 in the RC circuit 932 in figure 61 is 2.5 kohms, then from equation 110, the equivalent internal load Ri 216 in the system will also equal to 2.5 kohms. This resistance value described in equation 110 into equation 56 to get:
Equation 116
hut = kADC ~ k c * VLSB +Iφel = \kmc -kDAC] x lμA +hffset
Λli
Calibration
The method summarised in Table 9 can be used to calibrate the system depicted in figure 61. In this example, we first set kADC to an arbitrary reference level (kforce-ref) at 612. For the calibration process, in a first step a current reference (Iforce-ref) 936 of +200uA is applied to the system as shown in figure 62 and the value of kDAC as koAc- caii is recorded. In a second step, the output 204 is disconnect from any external sources/loads to make current Iout zero and the corresponding koAC as koAc-caβ is
recorded. The resultant calibration points are summarised in Table 20. These calibration values will be used in generating the lookup tables required in the force- voltage/force-current algorithms described herein below.
Table 20 - Calibration Points in the Example
Current Measurement
In a non-limiting implementation, the force- voltage-measure-current system described in connection with figure 37 and with reference to equation 57 can be constmcted using the VADC-Forcing circuit shown in figure 61. The resultant system is shown in figure 63. For this current-measurement system, the lookup table G 936 is a simple expression generated by calibration data. If the calibration points in Table 20 are substituted into equation 66 and the equation is simplified, the following can be obtained:
Equation 117 hut = G(k*DAc , kforce ) = [kforce - k Ac -7] x\μA
Knowing equation 117, the calibrated system in figure 63 can then be used to set an arbitrary voltage at output 204, from which a current Iout flows. In this example, let us assume that we need to force 1.28 V at the output 204 (VADC)- That means the corresponding value of kADC will be given by equations 72 and 103 as:
Equation 118
The resultant bit code k
DA
C can then be substituted in equation 117 to find the current value for I
out at output 204. For example, if an unknown current is applied and a k
DA
C of 866 is produced, the value of the unknown current I
out will be given by:
Iout = G(866,512) ' = [512 - 866 - 7] x \μA = -361μA
Equation 119
Iout = r (866,512) = [512- 866- 7l]xl t-4 = -361 ^
Current Generation
In accordance with a specific non-limiting implementation, an embodiment of the force-current-measure-voltage system shown in figure 23 (c) can be constructed using the VADC-Forcing circuit in figure 61. The resultant system is shown in figure 64. For this force-current system, the lookup table H"1 is a simple expression generated by calibration data. If we substitute the calibration points in Table 20 into equation 69 and 68, the following is obtained:
Equation 120
LU force) = ^
Equation 121
j. - ~l(k T Λ \ - k force 7
Knowing the result of equation 121, the system can then be used to set an arbitrary current at output 204 node. In this example, let us assume that a current if0rCe of 50uA must be forced at the output 204. The corresponding relationship in equation 121 can be simplified as:
Equation 122
'
ζDAC
= H V
ζADC ' lo t )
~ 7
After defining the lookup table H_1 from the calibration data, an external load R2 206 can be applied to the output 204 as shown in figure 64. With H_1 defined in equation 122, a current of value Iforce=50uA will be forced into the load R2 206 when the system is in equilibrium.
The general structure of the circuits shown in figures 63 and 64 is shown in figures 65 and 66 respectively. The set of circuit components labelled 1050 in figures 63 to 66 implement an equivalent functionality.
As shown in figure 65, the system includes an input 1012 for receiving a digital forced voltage signal, an output 204 suitable for releasing to an external load a signal approximating the digital forced voltage signal, a first circuit segment and a second circuit segment. The first circuit segment is between the input 1012 and the output 204. The second circuit segment is connected in a feedback arrangement between the output 204 and the first circuit segment and provides the first circuit segment with the voltage associated with the output 204. The first circuit segment includes a first digital-to-analog converter module 904, a difference module 934, an analog-to-digital converter module 906, a second digital-to-analog converter module 1002, an analog accumulation device 1000, an intermediate voltage point 214 and in internal load 216 between the intermediate voltage point 214 and the output 204. The first digital-to- analog converter module 904 processes the digital forcing parameter signal and generates an analog value of the digital forcing parameter signal. The difference module 934 processes the analog value of the digital forcing parameter signal as well as the voltage associated with the output of the circuit device and generates a difference signal. The polarity of the difference module 934 is reversed if internal load 216 is an inverting load. The analog-to-digital converter module 906 processes the difference signal and generates a digital representation of the difference signal. The second digital-to-analog converter module 1002 processes the digital representation of the difference signal and generates an analog representation of the
difference signal. The analog accumulation device 1000 generates a second voltage signal on the basis of the analog representation of the difference signal and applies the second voltage signal to the intermediate voltage point 214. The application of the second voltage signal to the intermediate voltage point 214 causes a change in either one of the voltage signal or the current signal at the output such that a voltage approximating the forced voltage signal is caused at the output 204.
In a first non-limiting implementation, the analog accumulation device 1000 includes an analog integrator module. In a second non-limiting implementation, the analog accumulation device 1000 includes a low-pass filter unit. It will be appreciated that other suitable equivalent devices instead of a low-pass filter unit may be used without detracting from the spirit of the invention.
The system shown in figure 65 also includes a current measurement circuit having a first input for receiving the digital representation of the difference signal generated by the analog-to-digital converter module 906, a second input for receiving a digital forced voltage signal from input 1012, a digital filter implementing accumulation functionality 900, search logic 936 and an output 938. The digital filter 900 is adapted for processing the difference signal received at the first input for generating' an average voltage value. The search logic 936 receives the digital forced voltage signal and the average voltage value from the second input and the digital filter 900.
The search logic 936 derives a certain current measurement on the basis of the digital forced voltage signal and the average voltage value such that, when the system is in equilibrium, the certain current measurement is indicative of an approximation of measurement of the current flowing at the output 204 of the voltage generating circuit.
The certain current measurement is then released at the output 938.
Figure 67 shows an alternative general structure of the circuit shown in figure 66. In this alternate configuration, the digital filter 900 of figure 66 present in the feedback path has been replaced by digital filter 1090 located between ADC 906 and DAC
1002. Digital filter 1090 may implement any suitable accumulation functional
element such as for example a digital integrator. The output of digital filer 1090 is provided to the search control logic 940. It will be appreciated that binary searches can also be performed by replacing the digital filter 1090 with a successive- approximation circuit (SAR) module without detracting from the spirit of the invention. The corresponding alternate stmctures associated with the circuit shown in figure 65 will be readily apparent to the person skilled in art upon a reading of this specification and as such will not be described further here.
Other specific examples of implementation of this invention are presented in C. K. L. Tarn, G. W. Roberts, "A Robust DC Current Generation and Measurement Technique for Deep Submicron Circuits", Proc. IEEE International Symposium on Circuits and Systems, Vol. 1, pp. 719-722, May 6, 2001. The contents of this document are hereby incorporated by reference.
SPECIFIC PHYSICAL IMPLEMENTATION
Those skilled in the art should appreciate that in some embodiments of the invention, all or part of the functionality previously described herein with respect to the circuit device and system may be implemented as pre-programmed hardware or firmware elements (e.g., application specific integrated circuits (ASICs), FPGA chips, ROM, PROM, EPROM, etc.), or other related components.
For example, the above described circuits may be incorporated in IC generally, diagnostic tools, IC testing equipment, on-chip testing and IC including on-chip testing functionality amongst others. Specific non-limiting examples of use of the above-described system include:
• the measurement of leakage currents in bonding pads of ICs; • continuity tests for bonding wires;
• parametric testing of logic output pins;
• parametric testing of output drivers (analog/digital);
• IDDQ testing of an IC or modules on an IC;
• parametric testing of resistors/diodes/transistors;
• parametric testing of voltage/current sources, bias networks or other analog components.
Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, variations and refinements are possible without departing from the spirit of the invention. Therefore, the scope of the invention should be limited only by the appended claims and their equivalents.