The simplest filters are those that comprise only passive components. These filters contain some combination of resistive (R), capacitive (C), and inductive (L) elements. The inductive and/or capacitive components are required because these elements present varying impedance to ac currents at different frequencies. As a refresher, you may remember that inductive reactance increases with frequency, whereas capacitive reactance decreases with frequency. Most passive filters used in the processing of biopotential signals are the resistive-capacitive or RC kind. This is because relatively large and heavy inductors would be required to implement filters at the low-frequency bands where biopotential signals reside, making inductive-capacitive (LC) filters impractical.
Despite their simplicity, RC filters are very common and effective in processing a wide variety of biopotential signals. Take, for example, the complete biopotential amplifier
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