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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.