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Gustavo Castro

# “O CMRR, CMRR! Wherefore art thou CMRR?”

D Feucht
1/12/2016 9:40:36 PM
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Blogger
More on CMRR
Good distinction, Gusatvo, between CMRR and common-mode gain.

For those interested in the basic derivations for these CM concepts for op-amps, see the Planet Analog article at

http://www.planetanalog.com/author.asp?section_id=3049&doc_id=563968

I wrote this article after working out the basic equations a different way than in the seminal IEEE paper on it by Pallas-Areny and Webster (ca 1991). Although in-amp ICs are modular and the user need not be concerned with resistor matching, for op-amp diff-amps, it is of major concern to know how CMRR will be degraded by resistor mismatch.

It took practically 30 years from when op-amps appeared for the basic CM derivations to be published!

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kendallcp
1/5/2016 6:45:37 AM
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Artist
CMRR in inamps and opamps
An awesome inamp, for sure!

I find a major area of confusion when I do analog talks and training is that CMRR is a specification you'll find both for inamps and for opamps.  The curve you see in an opamp datasheet applies to the opamp as a whole, independent of the configuration the user uses.  However, an inamp is typically used set to a particular gain, and it's meaningful to define a CMRR number for each gain.  An inamp with the resistive programming pins open circuit is functionally equivalent to an open-loop op-amp - but as you point out, a lot of inamps (especially home-made ones) won't necessarily display good CMRR figures used like that because they'll have internal features that compromise the common mode gain.

Personally I find common mode gain of an actual circuit to be the most useful number.  It's notoriously difficult to implement it in simulations, though, because many amplifier macromodels erroneously refer internal nodes to the simulator's 'GND' net when they shouldn't!

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