# Can someone help me understand why exactly high CMRR is so important, particularly in medical instrumentation like ECG?

So I have spent days reading about specifics of op-amps, instrumentation amps, and ECG circuitry and it just isn't clicking for me.

As I understand it, the common mode signal gets amplified and added to the differential output voltage. I don't see how this will be a problem since all it will do is provide a DC shift.

There seem to be several ways of compensating for DC shifts so why is it so important to have a high CMRR?

For example, in the following ECG circuit we already have the RLD and the output DC feedback loop for DC shift correction, so CMRR shouldn't be that big of a concern.

Despite this ECGs seem to have a minimum CMRR standard of 80dB.

• the common mode signal gets amplified and added to the differential output voltage That doesn't make sense when also to have a minimum CMMR standard of 80dB. So what is is it, amplification of common mode or rejection of common mode? The answer is that you want to reject the common mode. You want to amplify the differential mode signal as that's the signal you're interested in. Jun 9, 2021 at 20:53
• Suppose you have a large commonmode signal that is not rejected. The signal you want is differential and small. Both are appear at the output of this circuit. The output is single ended so there's no way to treat commonmode signals any differently from differential mode signals. So both are summed and appear at the output. So you'd have a large signal (from the commonmode) that you don't want and also a small signal signal that you do want. In extreme cases the opamp can clip due to the large commonmode signal making it impossible to retrieve the wanted signal. Jun 9, 2021 at 20:57
• in the following ECG circuit we already have the RLD and the output DC feedback loop for DC shift correction, so CMMR shouldn't be that big of a concern. Actually that DC feedback loop is part of the system which gives this circuit a high CMMR. Without that loop the opamp could clip due to it amplifying the (DC) CMM signal. By rejecting DC and CMM it will be possible to always amplify the differential signal despite large CMM signals. Jun 9, 2021 at 20:59
• Thank you. This did clear up a few things in my head! Jun 10, 2021 at 5:42