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Design >> Mixed-Signal Design >> CMRR in opamp
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Message started by zzhang on Jun 24^th, 2008, 8:36am

Title: CMRR in opamp
Post by zzhang on Jun 24^th, 2008, 8:36am

hi all:
Recently I am designing a telescopic opamp with gainboost.
I meet the problems in CMRR.
1. How to acquire the spec of CMRR? That is to say the influence of opamp's CMRR in their applicant, such as in data converter.
2. (May be the 2 is another saying of 1. :))
From text, we can get Acm = gm*R/ (1+2*gm*Rtail),where Acm is gain of commom mode, gm is transconduct of opamp, R is the
output res of opamp and Rtail is the output res of the tail current. With gainboost, the R will increase 100 times and so do the Acm, that is means that CMRR will decrease to 100 times. Do we need to consider the influence of gainboost?

Many thanks!

Title: Re: CMRR in opamp
Post by PKAT on Jul 7^th, 2008, 5:37am

Hi
1. How to acquire spec?
Ans: You need to know how much common-mode rejection your system can tolerate and from there assign a spec to it.

2. CMRR= Signal Gain/common-mode gain
Theoretically both Signal Gain and comm-mode gain will increase by same amount with signal gain increase (i.e. increase in R). So CMRR stays the same. You should do monte carlo with mismatch to get a feel for the reality.

Another way (bypassing Monte-carlo) of doing a crude (but reasonable) test, is to set the differential input to fixed full-scale and apply a common-mode AC input. Measure the differential o/p and there is your worst case scenario common-mode to differential gain.
Application of steady full-scale differential input assures some operating condition mismatch in differential system such that any common-mode disturbance is then sure to come out differentially to some extent.

Title: Re: CMRR in opamp
Post by jbdavid on Aug 27^th, 2008, 1:49am

I described a method for doing this simulation in ADC at a paper presented at ICU in 2001 - last time I checked it was still a white paper on the cadence website.. look for keywords "Functional Verification Operational Amplifier"
If its not available there.. send me a message.. I think I can find a pdf version of it..