Hi,

I am perplexed to find that the stability information (gain & phase margin) of my circuitry
are showing up differently depending on where I insert my probe element during the stb analysis
in Spectre.

The circuit is single-loop and I am using the loop-based algorithm, taking care to ensure that
the probe element selected is always in the loop. I use a 0V DC voltage source for the probe element.

Does anyone have any suggestion as to how I can avoid this problem?

Regards
Vivek

Hi Sheldon,

Thanks for the tips. Let me try these different methods. Unfortunately, I cannot post the schematic.
It is just a simple two-stage amplifier, where the compensation is at the output (output load >>>> any possible on-chip cap
by about 4-5 orders of magnitude). The first stage is a simple diff. gain stage, the feedback path has an R-divider.
I was experimenting with an additional level shifting stage between the first and the second stages.

There are several capacitive paths which might act as parallel paths at high-f, such as gate cap etc. So, I placed the probe
where I was sure that there were no signal paths in parallel to it.

The only possible reason I see is that the supply is not ideal (source impedance of ~ 2 Ohms), but I would not have expected
the kind of variation that I saw just because of this (~ 5 degree variation in PM).

But let me check the methods you describe.

Regards
Vivek

Hi Sheldon,

I am quite sure that the supply forms a second loop. The only problem I had was that I did not expect the amount of variation I got.
I was expecting more like 1-2 degrees change in phase margin, not in the range of 5-10. Perhaps my assumptions were incorrect then.

Anyway, I did AC and TRAN analyses to conclude that I had enough stability margin. In any case, STB analysis also gave me large enough
margin, even if the exact value was dependent on the probe location.

Thanks for your suggestions.

Regards
Vivek