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Simulators >> Circuit Simulators >> Characterize (simulation) CMRR of a single-ended OPAMP
https://designers-guide.org/forum/YaBB.pl?num=1235573924

Message started by Dipankar on Feb 25^th, 2009, 6:58am

Title: Characterize (simulation) CMRR of a single-ended OPAMP
Post by Dipankar on Feb 25^th, 2009, 6:58am

Dear All:
I want to get your opinions about the correctness of CMRR simulation methodology of a single-ended OPAMP (described below). I attach a figure for the bench I am using .

Here, vout/vcm=Ac/(1+Av-Ac/2)
(Ac=common mode gain, Av= diff-mode gain)
In ADE I use stb analysis to get Av (Av = loop gain in stb) and use XF/AC to get vout/vcm (same bench). Thus I know Ac and hence the CMRR.

I have doubt about the method as Ac is coming out to be quite large (even more than Av). Av is correct as I can verify the Av value from different bench (only stb analysis).

Please comment.

Title: Re: Characterize (simulation) CMRR of a single-ended OPAMP
Post by subgold on Feb 25^th, 2009, 8:18am

hi,

i think the cm input level is not correctly defined, there is no way to get the right equation for the calculation.

try to replace the voltage source in the feedback by a vdc source with vdc=0, do the same stb simu for Av, and use Vout/Vcm=(Av+Vc/2)/(Av-Vc/2) for Ac with ac/xf simu.