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In this paper, an analysis of power line interference in two-electrode biopotential measurement amplifiers is presented. A model of the amplifier that includes its input stage and takes into account the effects of the common mode input impedance ZC is proposed. This approach is valid for high ZC values, and also for some recently proposed low-ZC strategies. It is shown that power line interference rejection becomes minimal for extreme ZC values ( or infinite), depending on the electrode-skin impedance's unbalance ΔZE. For low ΔZE values, minimal interference is achieved by a low ZC strategy (ZC=0), while for high ΔZE values a very high ZC is required. A critical ΔZE is defined to select the best choice, as a function of the amplifier's Common Mode Rejection Ratio (CMRR) and stray coupling capacitances. Conclusions are verified experimentally using a biopotential amplifier specially designed for this test.