Direct interference canceling for two-electrode biopotential amplifier

A two-electrode biopotential amplifier, though simpler than a three-electrode amplifier, has problems rejecting 60 Hz power-line interference due to the imbalance of electrode–skin impedance. A direct interference canceling (DIC) scheme to reject 60 Hz interference for a two-electrode biopotential amplifier is proposed. The DIC scheme cancels differential-mode interference directly by adding to the interference to be canceled a feedback signal that has the same magnitude and frequency as the interference, but that has phase shifted 180°. When the DIC is started by closing a switch, transient interference appears, which decreases within several seconds. In steady state, the DIC amplifier rejects a very narrow bandwidth (BW) centered at the interference frequency. The proposed circuit was implemented and the DIC operation yielded 54 dB interference canceling with 0.6 Hz rejection BW.