Cri Azzolini
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Posts: 48
University of Parma, Italy
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Hi all!
I have got a problem during my AC analysis of op-amps. When I try to perform the analysis of the open-loop gain of the amplifier, the resulting Bode diagrams show very strange shape. In particular, both the magnitude and the phase of the open-loop gain show the typical low-pass shape but at medium-to-high frequency appear saw-toothed curves on the graphs. This saw-tooth profile is very sharp since it may cause the magnitude to change (randomly) of 10-20 dB in just few hertz. For this reason I do not think that is the real behaviour of the op-amp. These sharp sawtooth profiles lay upon the familiar low-pass open-loop curve but are very tiresome since, usually, they start around the unity-gain frequency where the phase-margin should be calculated. I tried to change the number of frequency points for the simulation but the result is the same. Probably it depends on the system I used for breaking the loop: indeed using ideal switches (AC position = open) the problem never appears. On the contrary, using AHDL or VerilogA switches, the saw-tooth usually appears.
The same problem results during PAC analysis of switched-capacitor circuits. Unfortunately the ideal switches in "analogLib" library do not work for PAC analysis so I have to use AHDL/VerilogA switches for PAC analysis.
Should I change some simulator options? Could anybody of you help me? Thanks in advance!
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