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What Is the Common Mode Rejection Ratio?

Jo Dunaway
Jo Dunaway

In telecommunications or professional recording applications, the common mode rejection ratio (CMRR) is a measurement of a tendency of a device to reject input signals to input leads. If there is a large voltage difference of relevant information between two signals or superimposed fluctuations of one of the signals on a paired line to an amplifier, this difference of fluctuation would read as a high CMRR. For example, if harmonics of a guitar overdub are being lost in the control room recording playback and that loss wasn’t heard during the live recording, the CMRR will point to where voltage differences show the loss of input to the recording machine. In other words, if a circuit is not keeping a constant output, it will be reflected in the common mode perturbation at the input level. The reduction of signal-to-noise ratio in recording situations is dependent on being able to detect these high common mode rejection ratio input signals, per frequency.

A high common mode input voltage to some op-amp models will cause a condition known as op-amp output latch-up, as it exceeded the allowable limits for the op-amp. As a rule, therefore, input voltage should never be allowed above power supply rail voltages because it can make the op-amp unpredictable or destroy its circuits. This can happen on power-up before capacitors have charged enough to filter out excesses and if a thoughtless tech didn't remove input signal prior to power up.

Woman holding a disc
Woman holding a disc

There can be a low common mode rejection ratio due to mismatch errors and tail current switching between N and P circuits in some low voltage operational amplifiers. Professional audio gear has balanced lines of differential input that have signals on two wires that are the negative of each other. Any noise on the wire will be common to both wires and they cancel each other, so the noise is rejected, is measurable in dB, and is called the common mode rejection ratio.

In medicine, battery-powered preamplifiers are attached to electrocardiograph machines and very high common mode rejection ratio induced for taking a special kind of electrocardiogram called a Laplacian. The purpose of this procedure is to find and study moment of activations in the heart’s myocardium without surgery. Through this real-time analysis it is possible to sense activation arrhythmias through the good signal-to-noise cancellation through sensors attached to the chest. In tomography, there is development work going on to use common mode rejection ratio at multifrequencies for thoracic impedance cardiography tests as well as total body bio-impedance analysis.

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