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Cri Azzolini

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Posts: 48
University of Parma, Italy

STB analysis of differential circuits
Feb 01^st, 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

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ACWWong

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Oxford, UK

Re: STB analysis of differential circuits
Reply #1 - Feb 1^st, 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 ?

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jbdavid

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Silicon Valley

Re: STB analysis of differential circuits
Reply #2 - Feb 1^st, 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..

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jbdavid
Mixed Signal Design Verification

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Cri Azzolini

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Posts: 48
University of Parma, Italy

Re: STB analysis of differential circuits
Reply #3 - Feb 1^st, 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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