In electronics, the common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a metric used to quantify the ability of the device to reject common-mode signals, i.e. those that appear simultaneously and in-phase on both inputs.
An ideal differential amplifier would have infinite CMRR, however this is not achievable in practice.
A high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong electromagnetic interference (EMI).
An example is audio transmission over balanced line in sound reinforcement or recording.
The CMRR is a very important specification, as it indicates how much of the unwanted common-mode signal will appear in the output, typically a measurement of some quantity.
[citation needed] For example, when measuring the voltage of a thermocouple in a noisy environment, the electrical noise from the environment appears as an offset on both input leads, making it a common-mode voltage signal.
The CMRR of the measurement instrument determines the attenuation applied to the offset or noise.
The key to achieving a high CMRR is usually the use of very precisely matched resistors (better than 0.1%) to minimise any difference in the amplification of the negative and positive sides of the signal.
The design of a microwave balun (single-ended to differential conversion circuit) defines the CMRR as the ratio of differential gain to common-mode gain in S-parameters, as follows:[1]
The CMRR of the balun represents the smallness of the gain and phase error between the differential outputs.
If the phase difference between the differential outputs of the balun is close to 180° and the amplitudes are equal, the CMRR will be high.