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Operational AmplifierBy : Sankalp - 02-Apr-2018 
What is CMRR in Op-Amp?
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Anil03-Apr-2018

Full form of CMRR is common mode rejection ratio.

Let us first understand the meaning of common mode signal. Basically Op-Amp is designed to only amplify the differential voltage at the input side. Common mode signal are those which are having same magnitude and phase and common to input terminals of Op-Amp and these are induced due to electromagnetic interference, unwanted transient voltages and appeared at the input side along with differential voltages. Whenever such types of signals are present, Op-Amp rejects these signals as much as possible and tries to produce the output of differential voltages only based on its CMRR value.

So CMRR is basically the ability of Op-Amp to rejects the common mode signals at the input. The Op-amp with higher CMRR will reject more unwanted signals. As Op-amp output is sum of common mode signal and differential voltage signal of the input terminal of Op-Amp. That’s why Op-Amp is designed with high CMRR.

CMMR= Ad/Ac, Where Ad= Differential voltage gain, Ac= common mode voltage gain (=0 for ideal Op-AMP)
Therefore CMMR is infinite for ideal Op-Amp.

Arpit02-Apr-2018

(1) Common Mode in Op-Amp is achieved when the input of both the terminals is common and thus we received the same input at both the positive and negative terminals of the Op-Amp.
(2) Rejection is defined as to what breadth the Op-Amp is capable of rejecting the common mode signals.
Common Mode Rejection ratio is defined as the ratio of Differential Voltage gain with respect to Common mode voltage gain. CMRR is a figure of merit for comparing/choosing an opamp

CMMR = Vd/Vc = Ad/Ac.
Where Vd & Ad are the differential voltage and currents respectively.
And Vc and Ac are the COMMON mode voltages and currents respectively.