From: Douglas C. Smith ([email protected])
Date: Fri Feb 04 2000 - 18:53:32 PST
Hi DougB and the rest of the crew,
I have a small two layer pwb with a dozen bypass caps on it that I use
for one of the demonstrations in my teaching. It is obvious from the
performance of that board that mixing the capacitor values is a good
idea. The board provides confirmation of your point number 4 below.
Mixing the values is especially important if the board has a regular,
repeated structure such as a memory board or other board with caps
located on grid coordinates. On such a board, my demo pwb, the caps
become part of an array of resonant circuits that have interesting
Doug Brooks wrote:
> Hi all,
> We have been putting some thought into this question and have derived,
> definitively we believe, some "truths" about this topic. They are
> summarized below. The first two or three are apparent and not really news,
> although we "prove" it from the normal formulas. The fourth has been
> verified as a "truth". We have presented all this in an article "ESR and
> Bypass Capacitor Self Resonant Behavior: How to Select Bypass Caps. We
> invite you to review the article.
> 1. As ESR goes down, the troughs get deeper and the peaks get higher!
> 2. The minimum impedance value is NOT NECESSARILY ESR (or ESR/n); it can
> and is lower than that!
> 3. The minimum impedance points are not necessarily at the capacitor
> self-resonant points.
> 4. For a given number of capacitors, better results can be obtained from
> more capacitor values, with moderate ESRs, spread over a range than with a
> smaller set of capacitor VALUES, with very low ESRs, even at well chosen
> specific self resonant frequencies.
> In order to verify the theory, we had to write a program that could, for
> any arbitrary set of capacitor (and inductance and ESR) values, find the
> minimum and maximum frequencies and impedance values. We have done that.
> The calculator is available for license.
> The article can be obtained from our web site:
> Follow the links to the ESR article.
> Doug Brooks
> See our updated message re in-house seminars on our web page
> Doug Brooks, President [email protected]
> UltraCAD Design, Inc. http://www.ultracad.com
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