A multi-electrode cardiac catheter has pairs of electrodes connected to sensing circuitry to produce a differential signal. The sensing output is sharply defined and self-normalizing. Preferably a catheter ablation tip electrode is an electrode of one pair, and the differential signal becomes non-zero...http://www.google.com.au/patents/US5341807?utm_source=gb-gplus-sharePatent US5341807 - Ablation catheter positioning system
