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Simulators >> Circuit Simulators >> Breaking loop with for stb analysis
https://designers-guide.org/forum/YaBB.pl?num=1369764095

Message started by RobG on May 28^th, 2013, 11:01am

Title: Breaking loop with for stb analysis
Post by RobG on May 28^th, 2013, 11:01am

I'm using the analogLib/cmdmprobe to find phase margin in a fully differential opamp (capacitive feedback) using stb analysis. Supposedly it doesn't matter where in the loop you insert it as long as you aren't inside a local loop... however when I put it at the front of the opamp I get a loop gain of 47 dB, whereas if I put it at the output of the opamp I get 45.3 dB. The response after 1GHz (2x my unity gain bandwidth) is also very slightly different. Otherwise the responses roll off at same rate and cross 0dB at the same point so the calculated phase margin is the same

I've looks for parasitics forming a local loop but just can't see any.

My interest is academic (I don't trust the models enough to believe either answer to that precision). Does it actually matter where the cmdmprobe is placed?

Best regards,
Rob

Title: Re: Breaking loop with for stb analysis
Post by Frank Wiedmann on May 29^th, 2013, 5:05am

The cmdmprobe is obsolete, you should use the diffstbprobe instead. The cmdmprobe only works properly for perfectly symmetric circuits.

Without a schematic, it's very difficult to tell where the difference you see might come from.