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cmdmprobe from analogLib

illaoi over 2 years ago

so I am looking at this component and wanna make sense of how it does calculate the loop gain.

from here we have

(1) in1 = out1

(2) (in1 - out1)*CMDM = in2 - out2, but from (1), that yields in2 = out2.

so far, all reasonable, since we broke the loop, and these variables are equal. What I need to understand is finj, so from PCCCS, I think we can write for the current that injects to out2,

(3) the current the "iprove vinj" has times CMDM?

i need help to understand

and how this essentially calculates the loop gain from stability analysis?

Also is CMDM=1 differential or common-mode stability?

Thanks!

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ShawnLogan over 2 years ago

Dear illaoi,

illaoi said:
(3) the current the "iprove vinj" has times CMDM?

Yes.

illaoi said:
Also is CMDM=1 differential or common-mode stability?

From URL:

support.cadence.com/.../techpubDocViewerPage

"Common Model Differential Model Probe
This is a Spectre subcircuit component used in Spectre stability analysis for measuring
differential stability. It measures the common-mode stability when CMDM is set to 1 and
differential-mode stability when CMDM is set to -1."

I am not sure of your need for this probe. However, as I detailed in the Troubleshooting article in this recent post:

community.cadence.com/.../fully-differential-opamp-stb-simulation

Cadence recommends the use of diffstbprobe for both common-mode and differential mode stability analyses. If you are looking to just understand the theory behind this schematic, Frank Wiedmann has an extensive note detailing the background (including references) of single and double injection as a means of determining loop gain and phase. His note is at URL:

https://sites.google.com/site/frankwiedmann/loopgain

I will not speak for Frank, but I believe he also recommends the use of the Cadence diffstbprobe in lieu of the cmdmprobe from his following response some years ago

at URL:

https://community.cadence.com/cadence_technology_forums/f/custom-ic-design/23009/cannot-perform-stb-analysis-in-cadence

I hope this helps a bit illaoi!

Shawn
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Andrew Beckett over 2 years ago in reply to ShawnLogan

Following on from Shawn's reply, you should not use cmdmprobe. It is obsolete. The big clue is when you try to instantiate it:

It was replaced because it didn't handle unbalanced differential loops, and the diffstbprobe is a much better way of doing this - then the choice as to whether you are measuring common-mode or differential-mode loop gain is a choice on the stb analysis form rather than being a parameter of the probe device.

Andrew
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Andrew Beckett over 2 years ago in reply to ShawnLogan

Following on from Shawn's reply, you should not use cmdmprobe. It is obsolete. The big clue is when you try to instantiate it:

It was replaced because it didn't handle unbalanced differential loops, and the diffstbprobe is a much better way of doing this - then the choice as to whether you are measuring common-mode or differential-mode loop gain is a choice on the stb analysis form rather than being a parameter of the probe device.

Andrew
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