CMRR test methodology and effect of open loop gain

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.

• Why would you need more resistors? Can't you just short the two inputs together? Dec 21, 2014 at 6:10
• Please refer to Fig 3 of the doc below for configuration: analog.com/static/imported-files/tutorials/MT-042.pdf Dec 21, 2014 at 6:40
• Using DC, how can you differentiate common mode errors from dc offset errors? I'd use AC everytime, maybe 100Hz or 1kHz. Dec 21, 2014 at 10:53
• I would recommend using both, your suggested method and the other method(s) suggested in the article. If you obtain the same results, you have validated your method. If not, then I would trust the method(s) used in the article. Dec 26, 2014 at 20:17
• Bob Pease is often good for these questions: ti.com/ww/en/bobpease/assets/bob_pease_lab_notes_2005.pdf Jul 12, 2018 at 3:33