Common-mode rejection(CMR) is a property of operational ampl ifiers and differential amplifiers that characterizes the suppression of identical signals at both inputs.
If completely identical voltages are applied to the inputs of a differential amplifier, so-called common-mode voltages, the output voltage of an ideal differential amplifier is zero, since the signals compensate each other. This behavior is essential for the suppression of interspersed noise signals that are simultaneously present in the same polarity on both inputs. Common mode rejection is frequency dependent and is expressed as a ratio of differential gain to common mode gain as common mode rejection ratio( CMRR). CMR( dB) = 20logCMRR.