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ideal integrator freq response dc gain problem

sjwprc

I used ideal components from analogLib to make an ideal integrator as

the freq response at top plate of cap is

which is strange. How there is dc gain, seems some res at parallel of Cap. Is this due to Cadence ?

BR

  • Andrew Beckett

    This is due to the fact that the vccs output is an ideal current source. That means the current needs to flow somewhere - and the challenge is that the capacitor would be open circuit at DC. So a 1V AC input results in 1mA needing to flow somewhere. Because the output of the current source is floating, a gmin resistor (by default 1e-12 Siemens, i.e. 1TOhm) would be inserted by Spectre to ground. That would mean that you get 1GV at the output and hence 180dB gain. 

    Ideal circuits are a challenge for many circuit simulators - if spectre had not inserted the 1TOhm resistor, then you'd have had a convergence problem - because there would be no solution that could be found - you can't have 1mA flowing and the output being floating. Normally the gmin resistors don't cause any issues in real circuits, but they would here.

    By the way, you can upload pictures to this forum rather than referencing them externally - that's better for the flow of information as they then appear in the email notifiers. Either drag in the picture into the post or use Insert->Insert Image/video/file

    Regards,

    Andrew.

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