I am trying to test the CMRR of opamp and so measuring offset variation across Common mode range in unity gain mode. I have gone through many articles on CMRR measurement. All those are suggesting a differential configuration of Opamp for testing CMRR.
Why shouldnt I test for offset across Vcm in Unity gain mode for CMRR ? In unity gain mode, varying the common mode voltage and measuring the offset as the difference of output and input, CMRR can be measured.
If I have to measure in differential mode, there will be 4 resistors besides opamp. The accuracy of these resistors will effect my measurement across Vcm, Vdd and Temperature.
Someone stated that the infinite open loop gain will effect the offset in unity gain mode across common mode range and give pessimistic values of offset as compared to differential mode. I tried with math to understand the effect of openloop gain on closed loop gain and found that the effect is the same whether it is unity gain or differential mode.
Please help me understand right methodology.