Common-Mode Rejection Ratio (CMRR) is a measure of the ability of an operational amplifier (op-amp) to reject a signal that is common to both inputs. Ideally, CMRR should be infinite, meaning that if both inputs fluctuate by the same amount, the output should not be affected. In practice, changes in common-mode voltage will produce changes in output. CMRR is defined as the ratio of the differential voltage gain to the common-mode voltage gain. A very high value of CMRR means that the differential gain is high and the common-mode gain is low. Thus, the higher the CMRR, the better the op-amp is at rejecting common-mode signals. A well-designed differential amplifier typically has a high differential gain and low common-mode gain, resulting in a high CMRR. The CMRR is often expressed in decibels (dB) . A CMRR of 10,000 (80dB) means that if the amplitudes of the differential input signal and the common-mode noise are equal, the desired signal will appear on the output 10,000 times greater in amplitude than the noise.

CMRR is an important specification of an op-amp, as it indicates how much of the common-mode signal will appear in the measurement. A high CMRR is crucial for accurately amplifying the differential input signal while minimizing the impact of common-mode noise. This enables op-amps to perform various tasks, such as amplification, filtering, and voltage regulation. Furthermore, a high CMRR also contributes to the overall stability and accuracy of an op-amp circuit, helping minimize errors caused by common-mode voltage fluctuations, temperature variations, and other factors.