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Simulators >> Circuit Simulators >> Large capacitor values cause problems
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Message started by Lieutenant Columbo on Sep 8^th, 2002, 1:06am

Title: Large capacitor values cause problems
Post by Lieutenant Columbo on Sep 8^th, 2002, 1:06am

I would like to use very large capacitor values (c=1e19) to push the lowpass filter corner freq to DC and so as not to interfere with my loopgain sim. Unfortunately, I cannot get Spectre to converge. I also tried 10e5 that did not work either. The highest value I can get it to work is 10e2, but at this value, my low-freq response is completely destroyed by the filter. What is going wrong? How can I fix it?

Title: Re: Large capacitor values cause problems
Post by Ken Kundert on Sep 8^th, 2002, 1:11am

Here is the problem. Spectre uses a nodal formulation, where we add together the admittance of each component at a node. The admittance of the large capacitors is jwC, or at 10GHz with c=1e19 the admittance is Y=j1e29. When you add such a large number to a small number on a computer with finite precision, the small number will be lost. Such a large admittance would completely swamp out any other admittance connected to the same node.

You can avoid this problem by replacing the large capacitors with the an instance of the following subckt.

subckt bigcap (p n)
parameters c
T1 (p n a 0) transformer n1=1 n2=1
C1 (a 0) capacitor c=c
ends

In this case, we separate the large capacitors from the other components and give them their own nodes. Then we count on the pivoting algorithms in the sparse matrix package to keep the errors under control.

Ken

Title: Re: Large capacitor values cause problems
Post by Ken Kundert on Sep 8^th, 2002, 1:13am

One more thing. There are better ways to compute the loop gain of an opamp. See chapter 3 of my book (The Designer's Guide to SPICE and Spectre).

Ken