Two-electrode biopotential measurements: power line interference analysis

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 Z_C is proposed. This approach is valid for high Z_C values, and also for some recently proposed low-Z_C strategies. It is shown that power line interference rejection becomes minimal for extreme Z_C values ( or infinite), depending on the electrode-skin impedance´s unbalance ΔZ_E. For low ΔZ_E values, minimal interference is achieved by a low Z_C strategy (Z_C=0), while for high ΔZ_E values a very high Z_C is required. A critical ΔZ_E 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.