Re: [SI-LIST] : Comment on Johnson's article

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From: Chris.H.Simon@gd-is.com
Date: Mon Nov 01 1999 - 05:54:54 PST


Arrigo,

I think that you are not missing anything important - you conclude that
small total parasitic inductance of the bypass capacitors is most
important, which is what the article concludes. Dr. Johnson further says
that once you have decided to use many small valued capacitors of a
specific size (and thus a specific parasitic inductance), why not select
the largest value capacitor that you can get in that package size? (Taking
into account cost, reliability, etc.) You pay no penalty (or one could
argue a small penalty) in parasitic inductance, but the total capacitance
in the system is larger which is a benefit at the lower frequencies of the
wideband systems that we design.

Chris Simon
General Dynamics Information Systems

Dear SI experts,

I have a question on the article posted on Dr. Johnson's web site at

http://www.sigcon.com/news/2_3.htm

entitled `Bypass Capacitor Layout'. At the end of the article
Dr. Johnson's states that since the only parameter that affects the
performance of bypass capacitors at high frequency is their parasitic
inductance, there is no point in using many small capacitors, rather
the highest value capacitor in the chosen package should be used.

Isn't this in contraddiction with the principle, also stated in the
book, that many small capacitors are a better choice since the total
parasitic inductance is lower? After the series resonant frequency
of 1/sqrt(LC) the impedance goes up with 20 dB/decade slope so the
only way to decrease it is by lowering either L or C, so both L and C
are important.

Am I missing something?

Thanks in advance,

-Arrigo

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