A novel AC-amplifier for electrophysiology: active DC suppression with differential to differential amplifier in the feedback-loop

A novel AC amplifier for electrophysiological measurements that uses a three op-amp based instrumentation amplifier (IA), and active suppression of the input offset was designed. The development followed the requirements to increase the common mode rejection ratio, thereby allowing the recordings of physiological signals in a noisy environment. In the proposed circuit, the high common-mode rejection ratio was achieved by using a high gain at the first stage of the IA (60 dB) and no passive components at the inputs. The feedback was achieved by means of a differential to differential amplifier placed in the input loop. This configuration provides an active suppression of DC-offset by feeding back an integrated form of the output that is equal to the input, yet of the opposite sign. The SPICE simulation of the novel amplifier with a gain of 80 dB and a high-pass cutoff frequency at 100 mHz are presented. The AC-amplifier could eventually be used for recordings of ECG, EEG, EMG, ENG, and other electrophysiological signals within virtual and telemedicine instruments.