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

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