# What is CMRR in Opamp

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Joined: Aug 16, 2016

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What is CMRR in Opamp
June 19, 2018 - 5:24pm

Why is this CMRR considered to be important, what should I check about this while selecting a Op-Amp? Offline

Joined: May 19, 2015

Replies: 212
Common Mode Rejection Ration
June 24, 2018 - 12:31pm

First let me give you the standard definition..

CMRR Stands for Common Mode Rejection Ratio, which can be defined as "The ration between the Differential Mode voltage gain (Ad) and the Common Mode voltage gain (Ac)"

Now, to understand things we should know what is this Ad,Ac and why is its ratio important in selecting an Op-Amp.

As we know op-Amp has two input pins, the difference in the voltage between these two pins is called Differential mode Voltage Gain (Ad) and the common voltage between these two pins is called Common Mode Voltage gain (Ac).

The output of an Op-Amp will always be the sum of Differential mode Voltage Gain (Ad) and the Common Mode Voltage gain (Ac).

So, for any Op-Amp it is always advantages to have high value of Ad and low value of Ac. A term that relates to these two voltage gains is CMRR.

Better an Op-Amp is the higher the value of Ad will be and thus the value of CMRR will also be high. The CMRR value will be so big and hence it is always measured in dB and its value should be high for good Op-Amps

For example if CMRR if CMMR is 10000, then CMMR in db = 20log(10000)=80db