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Simulators >> Circuit Simulators >> why doesn't ac signal add up?
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Message started by dog1 on Dec 10^th, 2013, 1:54am

Title: why doesn't ac signal add up?
Post by dog1 on Dec 10^th, 2013, 1:54am

Hi:

I am doing AC analysis for a circuit and the current around a node doesn't adds up. the circuit and the phase and magnitude information are attached and listed below. it can be seen that the cos and sin (real and imaginary) part doesn't add up. Does anyone know why is that?

PS: the table below list the magnitude and phase shown in the graph and also the cos=magnitude x cos(phase) and sin of those signals.
magnitude phase cos sin
port 2.47E-06 14.56 2.39E-06 6.22E-07
Ic 5.46E-06 73.784 1.52E-06 5.24E-06
Ids 9.38E-06 -108.706 -3.01E-06 -8.88E-06
IL 6.56E-15 -75.439 1.65E-15 -6.35E-15

Title: Re: why doesn't ac signal add up?
Post by boe on Dec 10^th, 2013, 2:06am

dog1,
I checked the values for port. They are consistent.
[edit]Did you check cmin and gmin? At which frequency is this anyway?[/edit]
- B O E

Title: Re: why doesn't ac signal add up?
Post by dog1 on Dec 10^th, 2013, 3:44am

Sorry for my ignorance but what is cmin and gmin? I tried to look them up in the cadence help but I don't think there is anything similar to what you means here.

I find something about nonlinearlity. So do you mean that those values takes care of nonlinearlity? I didn't know that. I always thought AC as linear analysis.

By consistence you mean that the table and figure are consistent or that the data follows KCL?

It's 60G

Thanks!

Title: Re: why doesn't ac signal add up?
Post by boe on Dec 10^th, 2013, 5:23am

Dog1,
Cmin and Gmin are minimum capacitance and conductance values that are added to all nodes for convergence reasons. You can choose the values in the options form. While you are at it, you should also check the error settings (particularly reltol, vabstol and iabstol).
- B O E

Title: Re: why doesn't ac signal add up?
Post by dog1 on Dec 10^th, 2013, 5:59am

Yes, I see that gmin is 1e-12, reltol is 1e-3, vabstol and iabstol is 1e-6 and 1e-12 respectively. I didn't really find cmin (in ADE--simulation--options--analog). Do you mean that I shall check all the voltage and current and make sure that they are much larger than vabstol and iabstol?

Title: Re: why doesn't ac signal add up?
Post by boe on Dec 10^th, 2013, 7:53am

Dog1,
Re Cmin, I was a bit too fast in answering: you find Cmin under the analysis options (it is used for tran, but not for AC - so it does not apply in your case).
Yes, all relevant voltages and currents should be much larger than v/iabstol - and the errors should be in the range of vabstol/iabstol or the relative tolerance reltol. Also, the leakage through Gmin should be much smaller than the difference you are looking for.
- B O E

Title: Re: why doesn't ac signal add up?
Post by boe on Dec 10^th, 2013, 7:58am

Dog1,
The consistency I mentioned previously was meant in terms of table/figure, not in terms of KCL.

Thinking a bit more about it, I suspect you have a parasitic in one of the models. I suggest you add a current probe (iprobe or voltage source w/ VDC=0) in front of each device and check if the current through those are equal to the port currents.

- B O E

Title: Re: why doesn't ac signal add up?
Post by dog1 on Dec 11^th, 2013, 5:53am

You are right! there is a great difference between the current in the prob and that shown at the port. But I always thought that the current flowing through the parasitic are also included in the port current. So is it not the case or is the difference is only the current flowing through gmin?

Thanks!

PS: sorry for the late reply. the maintenance messed up the model library and everything just recovered today.

Title: Re: why doesn't ac signal add up?
Post by boe on Dec 11^th, 2013, 9:55am

Dog1,
you said Gmin is 1e-12, so you'd need 1 MVolt (AC) to get 1 uAmp error current.
So I suppose your models contain some parasitics which are not included in the pin current of your devices.
- B O E
NB: At 60 GHz you need only a small cap to get 1 uAmp...

Title: Re: why doesn't ac signal add up?
Post by dog1 on Dec 12^th, 2013, 2:07am

Thanks a lot! I don't know that is possible before. A lot to learn:)

Title: Re: why doesn't ac signal add up?
Post by boe on Dec 12^th, 2013, 2:23am

Dog1,
the real reason may be a small charge conservation error (which would be equivalent to a small leakage cap)...
- B O E