# Instrumentational Amplifier - CMRR

Discussion in 'General Electronics Chat' started by Dritech, Nov 9, 2015.

1. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi. There are instrumentation amplifiers with different dB values. Can someone please explain in simple words what the dB value really means in an instrumentation amplifier?
I have read several articles, but I cannot really understand its function.

2. ### Papabravo Expert

Feb 24, 2006
10,338
1,850
There are two things to understand, dB and CMRR. Whenever something is measured in dB, it means that two things are being compared. In the case of the instrumentation amplifier I believe it is any common mode signal on the differential inputs to the output. If we have a common mode signal, of magnitude 1 Volt, and we want to know how much of that one volt signal might show up in the output we can look at the CMRR (Common Mode Rejection Ratio) for the answer. Suppose the number quoted is 70 dB.

$
-70 dB=20 \cdot log( \frac{v_o}{1} )
-70/20=log( \frac{v_o}{1} )
10 ^{-70/20} = v_o
vo = 316.2 \; \mu volts
$

The minus sign is because you are comparing a small output voltage $v_o$ to 1 volt. The logarithm of a number less than 1 is always negative.

Last edited: Nov 9, 2015
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