The Designer's Guide Community
Forum
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines.
Mar 29th, 2024, 7:44am
Pages: 1
Send Topic Print
Spectre modelling of an Ideal Capacitor (Read 9822 times)
MK
New Member
*
Offline



Posts: 7

Spectre modelling of an Ideal Capacitor
Jul 17th, 2006, 8:44am
 
I am trying to model a very basic system however it is not performing
as I expected.  I basically have an ideal current source (1 pA) feeding
into an ideal capacitor model (1uF) in Spectre.  I am expecting a
linear relationship of voltage vs time based on integrating the
capacitor current equation:

Ic = c*(dVc/dt)

integrating to get

Vc = (1/c)*Ic*t = Kt, where K = (1/c)*Ic

However, I am finding that when I attempt to model this in Cadence
using Spectre, the simulation saturates follows an exponential
relationship that saturates at the constant K value (1mV).  I cannot
seem to explain why it does not follow a linear function.  Does the
specre ideal capacitor model include resistance?  If so, is there a way
to remove that?
Back to top
 
 
View Profile   IP Logged
Ken Kundert
Global Moderator
*****
Offline



Posts: 2384
Silicon Valley
Re: Spectre modelling of an Ideal Capacitor
Reply #1 - Jul 17th, 2006, 10:46am
 
The ideal capacitor model does not contain a resistance. Because the node is floating, Spectre will add a 1TΩ resistor from the node to ground, and it warns you of this. This would only really be an issue if the drive current is very small (1mV/1TΩ).

Perhaps you could supply the netlist. The problem should be easy to find with that.

-Ken
Back to top
 
 
View Profile WWW   IP Logged
MK
New Member
*
Offline



Posts: 7

Re: Spectre modelling of an Ideal Capacitor
Reply #2 - Jul 17th, 2006, 11:31am
 
Hi Ken, I do not seem to get a warning of spectre inserting a 1TΩ resistor into my circuit, and it seems to be irrelevant what current I use, the voltage seems to saturate at the constant K value I specified earlier (so its dependent on the input current).

I have attached the netlist below.

// Generated for: spectre
// Generated on: Jul 17 12:25:42 2006
// Design cell name: amp_cap_test
// Design view name: schematic
simulator lang=spectre
global 0
include "spectre.init"

// Library name: experiments
// Cell name: amp_cap_test
// View name: schematic
C1 (Vc 0) capacitor c=10p ic=0
I3 (net10 Vc) isource dc=1f type=dc
V3 (net8 0) vsource dc=0 type=dc
V2 (net10 net8) vsource dc=5 type=dc

simulatorOptions options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 temp=27 tnom=27 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 maxnotes=5 maxwarns=5 digits=5 cols=80 pivrel=1e-3 ckptclock=1800 sensfile="../psf/sens.output" checklimitdest=psf

tran tran stop=1000 write="spectre.ic" writefinal="spectre.fc" annotate=status maxiters=5
finalTimeOP info what=oppoint where=rawfile
modelParameter info what=models where=rawfile
element info what=inst where=rawfile
outputParameter info what=output where=rawfile
designParamVals info what=parameters where=rawfile
primitives info what=primitives where=rawfile
subckts info what=subckts  where=rawfile
save C1:1
saveOptions options save=allpub

Thanks.
Back to top
 
 
View Profile   IP Logged
MK
New Member
*
Offline



Posts: 7

Re: Spectre modelling of an Ideal Capacitor
Reply #3 - Jul 17th, 2006, 12:40pm
 
Hi Ken, it seems that you were correct, by placing a 1TΩ resistor in parallel with my capacitor, I was able to reproduce what I was seeing.  By increasing the resistance, I can accurately model the ideal capacitor.

Thx!
Back to top
 
 
View Profile   IP Logged
Ken Kundert
Global Moderator
*****
Offline



Posts: 2384
Silicon Valley
Re: Spectre modelling of an Ideal Capacitor
Reply #4 - Jul 17th, 2006, 2:51pm
 
The messsage is ...
Quote:
Notice from spectre during topology check.
   No DC path from node `Vc' to ground, Gmin installed to provide path.

You can use the options statement to change the value of gmin. By default gmin=1p, which corresponds to a 1TΩ resistor.

-Ken
Back to top
 
 
View Profile WWW   IP Logged
Pages: 1
Send Topic Print
Copyright 2002-2024 Designer’s Guide Consulting, Inc. Designer’s Guide® is a registered trademark of Designer’s Guide Consulting, Inc. All rights reserved. Send comments or questions to editor@designers-guide.org. Consider submitting a paper or model.