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How sources and components are treated in DC analysis?

BaaB

Hi,

In DC analysis, capacitor is open and inductor is short. I would like to know what other things the DC analysis do (for example, how sources are treated,...). Could you tell me where can I find this? 

I tried search cdnshelp but the info is not there. It is an introduction about DC analysis.

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  • Andrew Beckett

    Not sure what you mean by "Cadence" (since Cadence is the company name, not the software), but spectre only has three independent sources: vsource, isource, and port. Various aspects of these are made available in analogLib in Virtuoso by other components, but they all netlist to one of these sources.

    As I said, it's very simple. It uses the "dc" value of the source (that's the dc parameter of any of these sources) if set, and if not, it uses the time-zero value of any time-varying source.

    Ken's book (the one I mentioned) simply says that the DC does this first:

    1. Configure all independent sources to be constant valued.
    2. Replace all capacitors with open circuits.
    3. Replace all inductors with short circuits.

    It's not a matter of "shorting" ac sources (current sources do not short at all). It's just stopping the independent sources from being time-varying and picking a value for that source (which I described how that works). I think you're probably over-complicating matters!

    BTW, dependent sources (e.g. vcvs etc) are used as-is - they are not treated differently during the DC analysis. It's only independent sources that have to remove their time-varying nature. This is hardly surprising, since there's no time in a DC analysis!

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    Not sure what you mean by "Cadence" (since Cadence is the company name, not the software), but spectre only has three independent sources: vsource, isource, and port. Various aspects of these are made available in analogLib in Virtuoso by other components, but they all netlist to one of these sources.

    As I said, it's very simple. It uses the "dc" value of the source (that's the dc parameter of any of these sources) if set, and if not, it uses the time-zero value of any time-varying source.

    Ken's book (the one I mentioned) simply says that the DC does this first:

    1. Configure all independent sources to be constant valued.
    2. Replace all capacitors with open circuits.
    3. Replace all inductors with short circuits.

    It's not a matter of "shorting" ac sources (current sources do not short at all). It's just stopping the independent sources from being time-varying and picking a value for that source (which I described how that works). I think you're probably over-complicating matters!

    BTW, dependent sources (e.g. vcvs etc) are used as-is - they are not treated differently during the DC analysis. It's only independent sources that have to remove their time-varying nature. This is hardly surprising, since there's no time in a DC analysis!

    Regards,

    Andrew.

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