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https://designers-guide.org/forum/YaBB.pl Design >> Mixed-Signal Design >> inconsistency between TRAN & PSTB https://designers-guide.org/forum/YaBB.pl?num=1192112136 Message started by vivkr on Oct 11th, 2007, 7:15am |
Title: inconsistency between TRAN & PSTB Post by vivkr on Oct 11th, 2007, 7:15am Hi all, I have an SC amplifier which has multiple feedback loops. I get incompatible results while simulating my amplifier with different methods. I did a PSTB analysis on the circuit to assess its stability, and found that although stable, it had a peaking in its response in 1 of the paths. I tried to see the impact of this by injecting disturbances at the offending frequency in voltage/current form in this loop, but failed to see any corresponding amplification of the disturbance. Then I tried the same but varied the frequency of the perturbation, expecting to see some variation in the amplitude of the perturbation at different points in the loop. However, contrary to results from PSTB which show a distinct frequency dependence, I got almost no variation in the multiple TRAN runs. I must say I am baffled. I use the traponly integration method in TRAN I have checked that the PSS analysis gives steady-state signal levels which match the steady state behavior in TRAN. The setups are identical except that the cmdmprobe from PSTB is replaced by the perturbation sources in the TRAN setup. I add a voltage of the order of 100 mV and current perturbation of about 100 nA, so as to avoid disturbing my circuit too much. I have used square waves, sine waves and damped sine waves to perturb the system, all with the same result. Any hints would be welcome on how to interpret this inconsistency between the 2 analyses. The circuit is stable as per both methods. Thanks Vivek |
Title: Re: inconsistency between TRAN & PSTB Post by tosei on Oct 16th, 2007, 7:57pm Hi Vivek, If you are dealing with multiple feedback loops you might know it is important the circuit nodes where to insert the cmdmprobe for running the PSTB analysis. Stability analysis - based on return ratios for voltage an current - as far as I remember is valid for multiple feedback only if: 1) All the feedback loops have a common break point at which all the loops are broken simultaneously 2) any single loop comprising the overall feedback is stable when analyzed separately from the rest of your system. Is there any change you might not be taking into account or violating any of these assumptions? If that was the case I imagine it would not be weird you get inconsistency between PSTB and TRAN analysis. Regards Tosei |
Title: Re: inconsistency between TRAN & PSTB Post by vivkr on Oct 16th, 2007, 10:57pm Hi Tosei, I do have a situation similar to what you outline. I have got 4 feedback loops (3 diff. and 1 common-mode). These do not all share a common point where they all might be broken with cmdmprobe. It is possible to break 2 of the loops together at 1 point, and 2 others at another. So, I did 2 different analyses. As for individual stability of the loops, I expect that they are all stable. I am aware that one needs to break all loops together, but I was expecting that I would atleast get a rough idea of the behavior even when this condition was not fulfilled. I was trying to check if the loops are well-behaved, and tried to break them 2 at a time as mentioned above. However, the frequency-dependent peaking in 1 of the loops, which is seen with PSTB is not apparent in transient. If I understand you correctly, I cannot correlate PSTB results with TRAN. In that case, my question is answered. Thanks very much for the clarification. Regards Vivek |
Title: Re: inconsistency between TRAN & PSTB Post by tosei on Oct 17th, 2007, 7:57pm Hi Vivek, I cannot make sure non-correlation between PSTB and TRAN is the result of running this analysis under your feedback conditions, but such discrepancy would not surprise me since you are not fulfilling the requirements that makes PSTB (and STB also) analysis a valid method. Regards Tosei |
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