A semiconductor integrated circuit includes a filter and a time-constant detecting circuit. The filter includes resistance elements; capacitance elements, each of which consists of a capacitance-value switching circuit that can vary the capacitance of the capacitance elements and operational amplifiers....http://www.google.com.au/patents/US6670846?utm_source=gb-gplus-sharePatent US6670846 - Semiconductor integrated circuit including a filter with adjustable cut-off frequency
