[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator
- From: "Peterson, James F (FL51)" <james.f.peterson@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 4 Aug 2003 12:18:44 -0700
Doug,
In regards to :
<snip>
1. The reason we wrote the calculator in the first place was because we
couldn't figure out how to make the frequency response calculation for an
arbitrary number of capacitors with arbitrary values, inductance and ESR
any other way. Then we figured others might have the same problem and would
like access to the same tool.
<snip>
We've been having excellent results doing this very thing using PSPICE.
We get complete control over ESR and ESL values, qty of caps, and their
values.
We plot the results as ohms and freq (log) - which requires a little
config...
regards,
Jim Peterson
-----Original Message-----
From: Doug Brooks [mailto:doug@xxxxxxxxxx]
Sent: Monday, August 04, 2003 1:42 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator
As the author of this calculator, I guess I'd like to throw in my 2 cents
worth!
1. The reason we wrote the calculator in the first place was because we
couldn't figure out how to make the frequency response calculation for an
arbitrary number of capacitors with arbitrary values, inductance and ESR
any other way. Then we figured others might have the same problem and would
like access to the same tool.
2. The calculator helps demonstrate (as does our article) that one thing
that is REALLY desired for a smooth impedance response is a continuum of
capacitor self resonant frequencies (not necessarily simply capacitor
VALUES). A large number of caps with the same self resonant frequency gives
you a single deep zero. Many self resonant frequencies spread over a range
give you multiple zeros, spreading out the response much more effectively.
MORE IMPORTANTLY, these zeros (and don't forget the poles in between) REACT
with each other to provide a smoother response. The calculator helps you
demonstrate and prove that.
3. Along the way, the results from the calculator provided the evidence
(unexpected at the time) that moderate ESR is better than low ESR. The
reason is that while low ESRs give you very deep zeros, they also lead to
very high peaks. Moderate ESR gives you an OVERALL better frequency
response.
4. Finally, the calculator does not address the issue of placement. My
input on that question is this: Placement doesn't much matter EXCEPT for
(a) the electrons must be able to get where they are needed in time
(traveling at 6"/ns) and (b) loop areas (including those around and through
caps) must be minimized for EMI reasons. Most people are aware of these
issues, and if they are taken care of, THEN placement is no longer an issue.
Hope this helps
Doug Brooks
>
Doug Brooks' new book, "Signal Integrity Issues and Printed Circuit Board
Design" has just been released by Prentice Hall. See details and ordering
info at www.ultracad.com
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