Ken Kundert wrote on Aug 16th, 2006, 11:23pm:I am well aware that using a very lowpass filter is a common trick for measuring loop gain. However, it is a poor approach that gives inaccurate results because it acts to break the loop at high frequencies and so affects the results by removing loading effects of the amplifier on the feedback network.
OK, agreed, but considering it is an LDO (sub MHz for the most part) and the load being broken off is the differential pair of the op-amp (small amount of C, not like the Cgs, Cgd of that big PMOS pass transistor. it should be close enough.
If you wanted to use this methodology, and include the amplifier input loading effects, you could use a second op-amp as a dummy load prior to the LPF. to include the loading effects. If really fussy, you can isolate the LPF out of the game with some VCVS behaviorals.
As with a lot of engineering, 5 ways will get it done, :) and a million ways to mess it up. :(
What would your approach be for opening the loop and maintaining the operating point? I looked at the paper that you mentioned, and it is comprehensive, but a bit of overkill for low frequency stuff in my opinion.
Also if the foundry models are off 30%, and you are trying to squeeze the the last 2% of accuracy out of the rest of the simulation?
tnx,
Jerry