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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> CMRR calculation https://designers-guide.org/forum/YaBB.pl?num=1128670701 Message started by bsrivastava on Oct 7th, 2005, 12:38am |
Title: CMRR calculation Post by bsrivastava on Oct 7th, 2005, 12:38am All, I need help me in testbench for CMRR calculation for an op-amp. I shorted the input with DC bias and gave common AC. Until certain freq the CM gain (Acm) is in fraction (output is less than input). From the definition of CMRR = 20log (Adiff/Acm), which is 20log Adiff - 20 log Acm = +ve value - (-ve value) = big +ve value seems unrealistic to me. regards, -Bharat |
Title: Re: CMRR calculation Post by sheldon on Oct 9th, 2005, 7:05am Bharat, Have you simulated CMRR over process variations? Some parameters are sensitive to mismatch, one example would be CMRR. Unless you account for the effect of mismatch on the parameter then the results will appear unrealistically high, particularly for fully differential designs. So add then the effect of mismatch to the models and run a Monte Carlo simulation, the results will probably look more realistic. Best Regards, Sheldon |
Title: Re: CMRR calculation Post by avlsi on Jan 31st, 2006, 2:12am Hi bharath, What ever u have done appears quite right to me. For Example, if u take Adiff/Acm,u must get a large positive value ( ideally infinity,as Acm must be ideally 0) if u take Acm/Adiff,u must get a value near 0. ( ideally zero,as Acm must be ideally 0) In practical,we take Acm must be around -60 dB. i.e., (around Acm=(Voc/Vic)=0.001). So now u verify ur results in this fashion. Bye, AVLSI. |
Title: Re: CMRR calculation Post by bunny on Feb 12th, 2006, 4:59am Sheldon, "So add then the effect of mismatch to the models and run a Monte Carlo simulation, the results will probably look more realistic." Can u please elaborate on this point and tell me exactly how to add the effect of mismatch in spectre. Any documentation in this regard will be highly thankful Thanq |
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