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The definition of CMRR in TI precision labs - Op Amps: CMRR

Yan Fan
Intellectual 360 points

Dear TI precision lab, 

I was watching the "7.1 TI precision labs - Op Amps: Common mode rejection ratio" section and got a question about the how to measure the CMRR. 

Theoretically, CMRR refers to the ration of Ad to Ac (differential gain to common mode gain). In the section, there were two definitions to measure the CMRR. One of them is CMRR =delta Vout / Vos, cmrr on Page 3. The other one is CMRR = delta Vos / delta Vcm on Page 4. Could someone shine some light on how these to definitions related to the theoretical definition of the CMRR = Ad / Ac?

Thanks,

B.F

  • 0
    TI__Mastermind 23050 points
    Hi B.F,

    Think about what your open loop differential and common-mode gains are. The differential-mode gain, otherwise known as Aol (open loop gain), is defined as a change in output voltage over a change in differential input, or Vos. So Ad = Vout/Vos. The common-mode gain is a change in output with a change in common-mode, or Vout/Vcm. Therefore, Ad/Ac = Vcm/Vos, but since we typically express CMRR in positive dB as a rejection ratio, you flip the relationship to Vos/Vcm.
  • 0 in reply to Zak Kaye
    TI__Genius 11135 points
    Bin

    We haven't heard back from you so we assume this answered your question. If not post another reply below, or create a new thread if this one has timed-out.

    Thanks
    Dennis