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mixed_signal

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finding current through capacitance in spectre (Cadence)
Oct 20^th, 2011, 11:33am

Hi,

I want to find the charging current of a capacitor connected to a push pull amplifier when the the input is sweeped from 0 to vdd.

I am using spectre in cadence where in ADE I am using DC simulation to sweep my DC I/P voltage along with Ac simulation so that I can see the current in the capacitor. I have added an AC source in series with DC source and I sweeped the frequency arbitarrily form 1KHz to 10KHz.

Unfortunately the current through capacitor is always 0. How can I see the current?

Thanks

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boe Community Fellow Offline Posts: 615	Re: finding current through capacitance in spectre (Cadence) Reply #1 - Oct 21^st, 2011, 1:27am Hi, did you save the current? Did you look at the AC current? Do you have a complex cap model? - B O E
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mixed_signal

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Re: finding current through capacitance in spectre (Cadence)
Reply #2 - Oct 21^st, 2011, 8:04am

Hi BOE,

I did not save current.

How can I look at Ac current?

I am using capacitor from analog parts in NCSU .25 um library. I dont know if it is a complex cap model.

I connected 0 volt voltage source in series to measure current. I connected 2 voltage sources. One in series with drain to drain of PMOS and NMOS and another in series with load capacitor. Remember it is a push pull amplifier with capacitor connected.

I could see the drain to drain switching current or direct path current or short cicuit current.

Thanks

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boe

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Re: finding current through capacitance in spectre (Cadence)
Reply #3 - Oct 21^st, 2011, 8:36am

Hi,
So you probe the currents through the voltage sources, right?
Here a few hints:
* First of all, saving of the (AC) currents must be enabled (ADE: Outputs/ToBeSaved or Outputs/SaveAll).
* You can use the Results Browser (ADE: Tools) to check if you have ac results after you ran your simulation.
* You can access your results e.g. with the results browser or using IF() in the calculator.

Does this help? If not, please post more details (e.g. what do you use to measure transient current, schematic, ...)
- B O E

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mixed_signal

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Re: finding current through capacitance in spectre (Cadence)
Reply #4 - Oct 23^rd, 2011, 12:37pm

Hi BOE,

The circuit and the i v/s vin curves i am trying �to simulate is attached. It needs to be mentioned that I1 is not equal �I2 except at vin=Vvg. I want to fing Vvg as shown in picture where I1=I2.

I am always getting I1=I2 and I=0 in my simulations. How can I get these curves?

Thanks

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pushpull.jpg

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boe

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Re: finding current through capacitance in spectre (Cadence)
Reply #5 - Oct 24^th, 2011, 1:38am

Hi Mixed_signal,
if you have a cap as load (or no load at all), you will always get I=0 for a DC stimulus because the simulator will determine the DC operating point such that I1=I2 (I1≠I2 is not in equilibrium unless the current I �can go somewhere). If you wish to get the plot you've posted, you need to define the output voltage.
E.g. if you fix the output voltage at VDD/2 with a voltage source, you will get the behaviour shown in your diagram.
Does this explain your results?
- B O E

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