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Does capacitor in analoglib have some leak current?

xxgenerall

Hi everyone,

I've simulated a very simple circuit, as follows.

It has a step-up signal as input. And signal "n2" is interesting.

Parents

  • The reason for this is that there is no DC path from n1 to ground (it's floating at DC), and so the simulator inserts a gmin resistor to ground. Since the default value of gmin is 1e-12 Siemens (1e12 ohms), you then have a 2 second time constant - if you simulate for 2 seconds you'll see it's down to 0.369mV (i.e. 63% of the way there, what you'd expect after a time constant).

    In this trivial circuit, you could set the option gmin to 0 (this is generally not a good idea, as gmin conductances are there to aid convergence - having absolutely no path to ground means that there are an infinite number of solutions for the DC operating point, since a floating node could be anything; a high value like 1e12 ohms is not unrealistic in terms of leakage).

    Regards,

    Andrew.

Reply

  • The reason for this is that there is no DC path from n1 to ground (it's floating at DC), and so the simulator inserts a gmin resistor to ground. Since the default value of gmin is 1e-12 Siemens (1e12 ohms), you then have a 2 second time constant - if you simulate for 2 seconds you'll see it's down to 0.369mV (i.e. 63% of the way there, what you'd expect after a time constant).

    In this trivial circuit, you could set the option gmin to 0 (this is generally not a good idea, as gmin conductances are there to aid convergence - having absolutely no path to ground means that there are an infinite number of solutions for the DC operating point, since a floating node could be anything; a high value like 1e12 ohms is not unrealistic in terms of leakage).

    Regards,

    Andrew.

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