Question regarding phase plot in TINA-TI for loop gain simulation of opamp.

Well Desiree, 

You can of course get the direct phase shift if you want to subtract from -180deg for phase margin, or, you can rotate the sense voltmeter to do that for you and plot phase margin directly - as they have done here for simplicity. 

Hello, thank you for your answer. Sorry but I am unable to understand where they have rotated the voltmeter. With respect to ground the voltmeter is positive.

Hello Desiree,   

Thank you for your inquiry about TINA-TI.  Since TINA-TI is a DesignSoft/TINA program, questions are handled by using the DesignSoft support page: http://www.tina.com/technical-support/  . 

The DesignSoft/TINA team is very helpful and this is the most direct way for TINA support.  Please advise by Reply to this case when the issue has been resolved.    

Regards, 

~Leonard  

In this case, they are injecting the signal back to the inverting input which gives you the 180deg phase shift. This approach is actually in error as it isolates the open loop output impedance from the feedback network - that often will give you the same result, but not always, here is a correction and more description in these articles, 

https://www.planetanalog.com/stability-issues-for-high-speed-amplifiers-introductory-background-and-improved-analysis-insight-5/#

And then more detail on VFA specifically, 

www.planetanalog.com/.../

Hi Desiree,

I think the rotation comes form observing the -input of OPAmp instead of the +input.

Kai

Hi Desiree again,

for the reason Michael mentioned, I prefer this method to perform the phase stability analysis:

desiree_phase.TSC

This method allows true differential input voltages. Or by other words, the +input does not need to sit on signal ground.

The only disadvantage is, that you must know the common mode input capacitances (C4 and C5) and the differential input capacitance (C6) of OPAmp. They have to be "mounted" away from the excitation signal, but inside the feedback loop, just as shown in the schematic above.

If the input capacitances are not given in the datasheet and cannot be read out from the Spice model, which is the case for some old legacy OPAmps, you can assume input capacitances of 10...15pF, as done in the above simulation.

Kai

And Kai, this is certainly very thorough - be sure to include any load on the op amp output when you do this, I often had numerical chatter on sims using 1T iso elements, using 99.97M sometimes helps that, 

Hi Michael,

yes, that's my experience too. L2, L3 and C3 have to be modified sometimes to get proper results.

Actually, I have "stolen" this scheme from TI's Tim Green

Kai

So yes, Tim and I did a lot of the TI content in these areas - I am consulting now and Tim is over at ADI having both kind of aged out of the TI preferred demographic. 

Thanks Kai, this is helpful.