OPAMP stability testing

[neo]

opamp ac analysis breaking feedback loop

hi, guys,
when testing the stability of opamp, when apply a jump to the input, either (-VSS to +VDD) or (+VDD t0 -VSS), if there is no ring or slow settling, can i make the conclusion that the stability is ok?
thanks a lot!

 

[pfd001]

stability opamp circuits

you should do ac simulation to get the phase margin

 

[neo]

cadence ac analysis break loop

actually, the ac simulation cannot get the accuraty PM only if one use the return ratio method, am i right?

 

[sachinagg77]

op-amp circuit stability - gain margin

For confirming the op-am stability perform the following:

1)AC analysis: Check the phase-margin, gain-margin
2)Transient analysis: Apply a small step input and see the output waveform. If the output settles within a specified period, the circuit is suitable for your application.

 

[neo]

to check opamp

For confirming the op-am stability perform the following:

1)AC analysis: Check the phase-margin, gain-margin
2)Transient analysis: Apply a small step input and see the output waveform. If the output settles within a specified period, the circuit is suitable for your application.

sachinagg,
1) u mean the generic AC analysis or the "break feedback loop" small signal simulation? For the PM of the OpAmp itself is not so important however the PM of the whole loop should be considered carefully, i think u mentioned only the PM of the opamp, not the loop itself.
2) u use "small step", u mean the small-signal? how do we know the slewing as well as nonlinear behavior resulting from large excursions, which cause the pole and zero freq to vary during the transient?

thanks

 

[sachinagg77]

op-amp testing for stability

Neo,

1)You are correct the loop needs to be broken at a suitable point and the loop needs to be checked.

2)With small step I meant that we should not use a step that goes from VSS to VDD [as mentioned in the original post]. As our aim is to check the op-amp circuit stability a small step is enough. For slew rate analysis, a larger step is necessary to see the large-signal effects clearly.

With Regards
Sachin

 

[neo]

opamp stability test circuit

then, the return ratio method is a must.
and, my further question is, if we do the transient test, can we ignore the involved return ratio method analysis?

 

[wangyuxin]

design and testing opamp

you can use cadence STB analysis tool

 

[neo]

opamp stability

the STB analysis is simply a cadence version of return ratio method, u also need to break the loop u care.........
sigh........

 

[davidwong]

op amp stability phase margin

the STB analysis is simply a cadence version of return ratio method, u also need to break the loop u care.........
sigh........

\

what is return ratio method ?

 

[renwl]

the STB analysis is simply a cadence version of return ratio method, u also need to break the loop u care.........
sigh........

\

what is return ratio method ?

There are detailed explain in the <<analysis and design of analog integrated circuits>> by paul r. gray.

 

[shaikhsarfraz]

Hi Friends
For getting the stability of the Op Amp we can do the following things
--> ac analysis can be done to get the phase plot and the gain plot.
--> stb analysis can be done with an iprobe in the feedback path of Op Amp
to get the absolute stability of the system.

 

[dragonsword]

I have the similar problem, did learn a lot

Added after 1 minutes:

the returned ratio method is originated in Gray's book?