ABSTRACT.Mismatched input electrode impedances can lead to noisy EEG and evoked potential recordings even if the electrode impedances are low. This paper presents a quantitative analysis of the phenomenon and illustrative examples.At each amplifier input, the recording electrode impedance and the amplifier's input impedance form a voltage divider. When input electrode impedances are unequal, a common mode signal that is present equally at the two locations on the patient's body appears at different magnitudes at the two (inverting and noninverting) inputs of the amplifier; thus, part of the common mode signal is amplified as if it were a differential signal. The effective common mode rejection ratio (CMRR) of the amplifier is reduced to a value that can be expressed mathematically as a function of the amplifier's ideal CMRR, its input impedance, and the impedance difference between the two input electrodes.Balanced input electrode impedances are particularly important during intraoperative monitoring, whe...