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

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From: Arrigo Benedetti (arrigo@vision.caltech.edu)
Date: Sun Oct 31 1999 - 23:56:05 PST


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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