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

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From: sweir (weirsp@a.crl.com)
Date: Mon Nov 01 1999 - 14:11:57 PST


No,

This is not a contradiction at all. You can reread it as saying that you
are better off having many small physical packages, each with very small
inductances in parallel, than fewer larger packages each with much larger
inductances in parallel. Dr. Johnson's point about the capacitance is that
charge storage is what the capacitors were put there for in the first
place, so once the package is set, get as much as you can. With the
possible exception of trying to tune resonance, this practice is commonly
adhered to.

Regards,

Steve.
At 11:56 PM 10/31/99 -0800, you wrote:
>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
>--
>Dr. Arrigo Benedetti e-mail: arrigo@vision.caltech.edu
>Caltech, MS 136-93 phone: (626) 395-3695
>Pasadena, CA 91125 fax: (626) 795-8649
>
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