Common-mode rejection ratio

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The common-mode rejection ratio (CMRR) of a differential amplifier (or other device) is the rejection by the device of unwanted input signals common to both input leads, relative to the wanted difference signal. An ideal differential amplifier would have infinite CMRR; 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. An example is audio transmission over balanced lines.

Example: operational amplifiers[edit]

Typical instrumentation amplifier implementation, designed to have a high CMRR.

An operational amplifier (op-amp) has two inputs, V+ and V-, and an open-loop gain G. In the ideal case, the output of an ideal op-amp behaves according to the equation

V_\mathrm{out} = (V_+ - V_-) \cdot G_\mathrm{openloop}

This equation represents an infinite CMRR: if both inputs fluctuate by the same amount (while maintaining a constant difference V+ - V-), this change will have no bearing on the output. In real applications, this is not always the case: the lower the CMRR, the larger the effect on the output signal, following the general equation

V_\mathrm{out} = (V_+ - V_-) \cdot G_\mathrm{openloop} \pm \frac {V_\mathrm{cm}}{ 10 ^ {\frac {CMRR}{20}}}

Where VCM represents the common-mode voltage at the inputs, or (V+ + V-)/2.

The 741 (a common op-amp chip) has a CMRR of 90 dB, which is reasonable in most cases. A value of 70 dB may be adequate for applications which are insensitive to the effects on amplifier output;some high-end devices may use op-amps with a CMRR of 120 dB or more.

So for example, an op-amp with 90dB CMRR operating with 10V of common-mode will have an output error of ±316uV.

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