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https://designers-guide.org/forum/YaBB.pl Simulators >> Circuit Simulators >> STB analysis of differential circuits https://designers-guide.org/forum/YaBB.pl?num=1170321868 Message started by Cri Azzolini on Feb 1st, 2007, 1:24am |
Title: STB analysis of differential circuits Post by Cri Azzolini on Feb 1st, 2007, 1:24am Hi all, I am trying to perform a stability analysis on a fully-differential op-amp; as suggested here and there, I am using the STB analysis with the "cmdmprobe" (analogLib) as probe element inside the differential loop. The circuit is shown below and is very simple: the op-amp cell includes a continous-time CMFB. The Bode diagram obtained by means of Result->Direct Plot->Main Form is shown below: I tried different arrangement and options but the result is the same: as far as I can see the Bode plot is absolutely without sense. Actually I used STB with cmcdprobe with many different circuits but such analysis seldom returns reliable results. Do you have suggestion? Do I make a mistake in the setup? Many thanks in advance, bye! Cri |
Title: Re: STB analysis of differential circuits Post by ACWWong on Feb 1st, 2007, 1:48am have you tried spectre -h stb ? there is a section on a differentail opamp anaylsis using cmdmprobe (diffprobe in the help). It shows how setting CMDM changes the type of analysis you are doing. By default it is 1 so this means common-mode loop gain, and -1 means differential mode loop gain. cheers aw PS. about you opamp, the inputs and outputs are tied near to VDD by 200ohms ? |
Title: Re: STB analysis of differential circuits Post by jbdavid on Feb 1st, 2007, 2:16am You also might want to use RESISTORS in your feedback network.. as I don't know that what you have is really a unity gain configuration for a differential amp.. (see the Functional Verification of a differential operational amplifier (the cadence white paper) that paper was written before STB analysis existed. so it doesn't cover that, but I think you need a real unity gain circuit.. |
Title: Re: STB analysis of differential circuits Post by Cri Azzolini on Feb 1st, 2007, 2:22am Thanks all, ACWWong was right, I set CDDM=1 so the common-mode loop gain was measured; it seems I did not understand correctly how the "cmdmprobe" works: now it is clear and the results are more acceptable. Many thanks, Cri PS: ACWWong, the output stage of the op-amp has an open-drain configuration whereas the voltage at the input nodes are adjusted by an internal level-shifter; sorry but I was in a hurry when I prepared the figures. Anyway, thanks for the suggestion. |
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