michaelFCT wrote on Mar 29th, 2010, 7:59am:Sorry about the lack of detail, but I am unable to disclose any information about the amplifier and the CT-CMFB.
The SC-CMFB can be found in "Choksi, O.; Carley, L.R.; , "Analysis of switched-capacitor common-mode feedback circuit," Circuits and Systems II: Analog and Digital Signal Processing, IEEE Transactions on , vol.50, no.12, pp. 906- 917, Dec. 2003 doi: 10.1109/TCSII.2003.820253" (Fig. 10).
By cm probe do you mean the "CMDMPROBE" block? If yes, this block is from Cadence's analogLib. If no, I don't know what you mean.
I have already sized the transistors for this amp. and got it work as I wanted (DM and CM were stable). I then tried a different sizing and got the ringing CM.
I want to know, why I can't detect it with STB analysis. It obviously is because of the sizing, but I must be able to detect the instability with an AC simulation of some kind.
Michael.
ok, i am not sure if what i am about to say is your case, just want to give you a hint.
in your very first post you said "thus breaking all loops in the circuit", but you should be a little cautious on this claim. you have to distinguish between many cases:
1) there are multiple feedback loops in parallel,
2) there is a global feeback, composed by some inner feeback loops,
3) there are cross-coupled feedback loops
4) more complicated situation (e.g. feedforward involved)
5) ....
the claim you referred is always valid only for case 1, and you circuit seems to be case 2. i am afraid what you got from stb analysis is just the stablity summary of the inner loop. but obviously there are lots of simple ways to verify this. for example you can check what you get when you move the position of the probe, or simulate the inner loop alone (with the correct loading), etc.