RE: [SI-LIST] : Decoupling caps selection

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From: Thomas Jackson (tjackson@fmi.fujitsu.com)
Date: Fri Oct 27 2000 - 16:50:36 PDT


The "Impedance vs Frequency" data that the capacitor manufacturers give is
just that. It shows at what frequencies their parts cease being capacitive
and start becoming inductive. Note that for most common electrolytic
capacitors, that is in the ones to tens of MHz range even though current
clock rates go into the 100s of MHz and frequency content of the edge rates
go into the GHz.

This is why short-wide capacitor packages like 0508 are more desirable than
the common narrow-tall packages like 0805. The short-wide capacitors stay
capacitive up to about twice the frequency as the comparable narrow-tall
equivalent.

The idea is to minimize inductance over the frequencies of interest (up to
the edge rate "knee" frequency). One can also minimize the inductance by
minimizing the current loop area from the power plane to the capacitor and
back to the ground plane. Again, one can get about a factor of 2
improvement by the correct layout.

Edge rates are, primarily, functions of the devices being used. While the
semiconductor manufacturers, generally, don't want to add it to their specs
and, therefore, test for it, general edge rate information is available in
the SI literature for common logic families and it is buried in device IBIS
models such that simulators can use it. I have not had any luck calculating
edge rates from IBIS models by hand.

I hope this helps.

Thomas L. Jackson, P.E.
Staff Product Development Engineer
Network Access Development
Systems Solutions Group
FUJITSU MICROELECTRONICS, INC.
3545 North First Street
San Jose, CA 95134-1804
telephone: (408) 922-9574
facsimile: (408) 922-9618
http://www.fujitsumicro.com

-----Original Message-----
From: Dilpreet Saini [mailto:DSAINI@altera.com]
Sent: Friday, October 27, 2000 3:15 PM
To: SI List (E-mail)
Subject: [SI-LIST] : Decoupling caps selection

Hi all,
        I was wondering if the selection of the decoupling capacitor of a
circuit is MAINLY governed by the edge rates of the switching signals or by
the frequency of their switching. I believe the answer is later cause all
the documentation that I have seen on the dcoupling cap selection so far do
not have any mention of the edge rates. Most of the capacitor manufacturer
have "Impedance Vs Frequency" charts in their specs/catalogs. I am assuming
that the Frequency that they are referring to is the frequency of operation
of the circuit. Any ideas/comments are greatly appreciated.

Dilpreet

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