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Power signals in Virtuoso

marten

Hi,

I need to understand, how spectre calculates power signals. E.g. imagine a simple CMOS inverter instance I0, when I manually calculate and print the power signal with P=I*Vdd, I get a different signal than by just printing I0:pwr. Obviously, the internal power signal contains a different current than the one used in my "hand calculation". There I measured the current dropping out of power supply voltage source (V0:PLUS).

Could somebody please explain, how the power signals (I0:pwr) get calculated internally?

Cheers

  • Spectre only calculates the static power (it's an operating point parameter - so normally it's used for DC), and not the power in capacitors/inductors. When you save :pwr for either an element or a subckt during a transient, it's saving the static (DC) power at each time point, and for a subckt is just the sum of all the :pwr outputs for the individual devices within that subckt. If the component doesn't support saving :pwr (see "spectre -h resistor" and compare with "spectre -h capacitor"), then you will be getting no contribution from those devices. The same would be true of any Verilog-A model that doesn't explicitly publish its power.

    So I would say :pwr is of rather limited usefulness in a time-varying analysis - it's really just telling you the "DC" power consumption at a variety of operating points, which I doubt is very useful - it's OK to use this at DC itself, and you can then look at the pwr contribution for individual devices - but for transient, I'm not convinced...

    Regards,

    Andrew.

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