Enhancing interference rejection of preamplified electrodes by automated gain adaption

The use of preamplified electrodes in electrocardiography (ECG) or electroencephalography (EEG) has a numbe of benefits, most importantly, the noise reduction through preamplification and the interference reduction through buffering (Fernandez and Pallas-Areny, 1997). Normally, preamplified electrodes are not used because the mismatch in amplification due to component tolerances leads to a reduced common mode rejection ration (CMRR) (Pallas-Areny and Webster, 1991). In this paper, we introduce a new technique to restore the CMRR. We do this by adapting the gain of the differential amplifier (DA) following the preamplified electrodes. The technique is based on measuring the transfer function of a chosen common mode signal. The gain of the DA is adjusted by a closed loop controller to maximize the CMRR. With this technique, there is no adverse effect on the ECG or EEG. In this way, the control loop can be operated continuously if needed.