Re: [SI-LIST] : RE: bypass caps

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Mon Jun 05 2000 - 15:13:54 PDT


Jay,

In my opinion, Mr. Brooks addresses this in the
most recent issue of Printed Circuit Design with
his article _ESR and Bypass Capacitors_. No?

And if I may, extending this concept a little further,
placement of the caps depends upon the resonant points
of your board. Selection of the caps depends upon the
resonant frequencies of those points. At the Denver
EMC symposium a few years back, there was an excellent
presentation by *I think* Sun on this topic. Apologize
if I'm way off on that.

- Doug

Jay Marano wrote:
>
> I'm looking for any feedback from those who have
> read the white paper on the Ultracad web site
> regarding ESR and Bypass self resonant behavior.
> One of the things it demonstrates is that for a
> given number of capacitors, better system impedance
> results are obtained from using more cap values,
> with moderate but not super low ESR, spread over a
> range than with a smaller set of cap values, with
> very low ESR, at well chosen self-resonant frequecies.
> I understand the theory behind his analysis, but it
> raises an interesting question: In the frequency range
> between the bulk capacitors and the knee frequency of your
> system, is it better to purposely use wider tolerance caps
> and use a smaller number of values, or buy more tighter tolerence
> values and rely on the nominal values to smooth out the system
> impedance? Any feedback on this analysis would be appreciated.

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