spectre stb analysis: DC phase is 0 instead of 180 degree

[Wu_uW]

Hello all,

I have a strange problem:
I run stb analysis for my circuit (opamp buffer) for all corners. I found for some corners my phase plot start at 0 instead of 180 degree, while the location for the poles are still correct, so I can still have correct PM.
Has somebody also experienced same thing? and what is the cause?

thanks in advance,

 

[dick_freebird]

I've seen this, I don't understand why, so I don't trust the tool.
I do not have the same problem with the same circuit using the
more primitive methods (a vcvs to "sniff" vin(diff), calculator
plot the mag out/in and phase out-in).

 

[Wu_uW]

Thanks for the reply.
I also got correct result with primitive way, so I believe the circuit is ok.
Just don't know the magic of spectre stb analysis.

regards,

I've seen this, I don't understand why, so I don't trust the tool.
I do not have the same problem with the same circuit using the
more primitive methods (a vcvs to "sniff" vin(diff), calculator
plot the mag out/in and phase out-in).

 

[shilpashetty]

Hi
Can you attach your schematic? are you simulating in differential or single ended ? what probe are you using?

 

[Wu_uW]

Hello,

My schematic is very simple: a class AB output amplifier connected as a buffer.
single ended, iprobe connected in feedback. nothing else.