FreeLists / si-list / [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator

[Doug Brooks <doug@xxxxxxxxxx>]

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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