Common-mode rejection ratio

The common-mode rejection (English common- mode rejection ratio, short CMRR ) specifies in electronics, how little the output voltage changes when the two input voltages of an electric differential amplifier by the same amount ( ie in the " common mode " ) change. Ideally, then it should not change the output voltage of the differential amplifier (for example, an operational amplifier ), as it depends only on the difference of the two input voltages. This aim is largely achieved through extensive instrumentation amplifier.

Basics

While the voltages U ideal differential amplifier is determined by the equation

Described. Here, the differential gain ( " push-pull amplification "). When real difference amplifier, the shares of the common-mode gain added:

Up and Un wherein the voltages at the two inputs, the gain of the difference and Adiff Agleich undesired common mode gain.

The CMRR is defined as the ratio of the two gains and has no unit:

This ratio is often expressed in decibels:

An equal change in voltage on the two inputs thus results in not full common-mode rejection to a slight change in the output voltage. The better common mode rejection, the greater may be fluctuations in the offset voltage or offset voltage, which is applied as a common mode signal at the inputs of the amplifier.

In most applications one would like to amplify only the difference voltage between the two inputs and independently of a common mode signal (UCM - CM for common mode ) to be. Has practical significance to suppress example, in the balanced signal transmission to common mode noise, such as hum, a very high common mode rejection.

Another feature that the feature common mode rejection expresses analogous dependence on changes in the supply voltage is referred to as the supply voltage feedthrough (English Power Supply Rejection Ratio PSRR ).

Example

In operational amplifiers CMRR values ​​are up to 130 dB common. This means that changes in the top 10 uV -mapped circuit with a common-mode voltage of Ue = 30 V, the output voltage Ua = only.