Interference-rejection characteristics of biopotential amplifiers: a comparative analysis

The circuit parameters contributing to power line interference in ground-referenced, two- and three-electrode biopotential amplifiers, both isolated and nonisolated, are reviewed. The effects of external interference on different amplifiers are compared by using the ´effective coupling impedance´ concept. Next, an analysis of the effect of imbalanced input impedances is carried out. It is concluded that the interference in an isolated amplifier is not always lower than in a nonisolated one, and that it must be reduced by an adequate sharing between the CMRR (common mode rejection ratio) and the IMRR (isolation mode rejection ratio); that an increase in common mode input impedance always reduces interferences in three-electrode amplifiers but not in two-electrode amplifiers; and that in two-electrode amplifiers, not only is the matching of input op amps important, but also the tolerance of components in first-stage circuits.