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Is there a way to measure the equivalent capacitance from a node to ground in a circuit?

BaaB

Is there a way to measure the equivalent capacitance from a node to ground in a circuit?

I see that the node to ground in captab only reports the capacitance value of an actual capacitor (if exists) from that node to ground not the equivalent capacitance.

Thank you.

Parents
  • Andrew Beckett

    The result is "right" but it's not doing what you're expecting. It would rather have helped if you had explained at the beginning you were aiming to measure the effect of Miller capacitance, which is not what happens with captab. In essence the Miller effect is telling you the equivalent input impedance caused by an impedance between the input and output of a gain stage - by "effective" I assumed you meant the capacitance to all nodes as if it was referred to ground.

    Given that the Miller capacitance will be frequency depended (because the gain will be frequency dependent), this isn't something the simple captab analysis would give you, which is based on all the capacitances in the devices rather than any consideration of the gain (it doesn't run at a particular frequency, so it couldn't do that anyway).

    I've never thought about how you would simulate to compute this for all nodes - I'm sure you could simulate it for a specific node by injecting a sharp transition and measuring the effective time constant (or something like that?). Most of the time as a designer, I've just been happy to take advantage of it to allow compensation capacitors to be smaller, rather than worrying about simulating what the equivalent input capacitance would be. 

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    The result is "right" but it's not doing what you're expecting. It would rather have helped if you had explained at the beginning you were aiming to measure the effect of Miller capacitance, which is not what happens with captab. In essence the Miller effect is telling you the equivalent input impedance caused by an impedance between the input and output of a gain stage - by "effective" I assumed you meant the capacitance to all nodes as if it was referred to ground.

    Given that the Miller capacitance will be frequency depended (because the gain will be frequency dependent), this isn't something the simple captab analysis would give you, which is based on all the capacitances in the devices rather than any consideration of the gain (it doesn't run at a particular frequency, so it couldn't do that anyway).

    I've never thought about how you would simulate to compute this for all nodes - I'm sure you could simulate it for a specific node by injecting a sharp transition and measuring the effective time constant (or something like that?). Most of the time as a designer, I've just been happy to take advantage of it to allow compensation capacitors to be smaller, rather than worrying about simulating what the equivalent input capacitance would be. 

    Regards,

    Andrew.

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