# How to quantify the effect of the common mode input for a given CMRR?

The definition of CMRR is 20×log10(Aol/Acm) where Aol is the open loop gain and Acm is the common mode gain.

If we have a buffer with 20dB CMRR opamp and if we apply 1V differential input along with 1V common mode input as shown in my hand drawn example:

In this case we dont know the open loop gain but we know the CMRR. What will be the total output voltage? In other words how to quantify the contribution of the 1V common mode input to the output voltage. We are using feedback here so would that effect CMRR?

Edit:

Look at a data acquisition input parameters:

CMRR is given as 92dB but Aol never mentioned. Does that mean we cannot quantify Acm?

• What is the differential gain than (Aol)?
– G36
Commented Apr 13, 2019 at 15:24
• Im confused because there is feedback and the rest Commented Apr 13, 2019 at 15:25
– G36
Commented Apr 13, 2019 at 15:25
• Is my buffer diagram with differential amplifier wrong? Commented Apr 13, 2019 at 15:34
• Yes, it is wrong. Your buffer has only one input (noninverting).
– G36
Commented Apr 13, 2019 at 15:36