RE: [SI-LIST] : Capacitor dielectric type

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From: Ken Cantrell (Ken.Cantrell@srccomp.com)
Date: Fri Nov 17 2000 - 13:19:44 PST


Tk,
One of the other guys e-mailed us some capacitor links, my thanks again to
whomever. I think the Kemet software was the best, and recommend it for
getting the visual feel for the differences in dialectrics and the effects
of external series inductance and resistance. Then I plug those values into
Doug Brooks's package (Ultracad.com, ESR software*), to see the effects of
paralleling all those caps. A real eye opener. The NPO/COG caps with low
ESR's ring like crazy, yielding sharp discontinuities. Ugly looking. This
supports Richard's e-mail below. My input is simulated only at this time,
and it was great to hear that someone had lab data.

Ken

*The software is CHEEEP! @ $75. Keep in mind it is a dedicated package, just
does this one thing. It's my favorite screwdriver now. The only thing I
would like to see added is print/cut&paste of the graphical display. Doug,
when you gonna do that?

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of RMELLISON@aol.com
Sent: Friday, November 17, 2000 1:09 PM
To: tkjeon@vps6.lsil.com; si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Capacitor dielectric type

Tk,
I've simulated and verified in the lab the effects that you are worried
about. I have seen that X7R caps provide a smoother decoupling, due the
fact
that the higher ESR redcuces the Q of the circuit. The capacitors and their
internal inductances, as well as the inductance of the power planes, can
make
a field of resonators which can interact severely to produce a poor
decoupling network. For instance, one power-ground sandwich had an
impedance
that rose above 25 ohms at 60 MHz, as a result of using NP0 dielectric.
Obviously, in the case of a very large di/dt requirement, the NPO capacitor
is going to do a better job of supplying the current due to it's lower ESR.
An alternate approach would be to supply the current through many more X7R
caps.
I modelled the power-ground sandwich as a group of horizontal and vertical
interconnecting transmission lines using Pspice. Lab results using a
network analyzer proved the accuracy of the model.
Richard Ellison, consultant
RMEllison@aol.com
ph. 972-569-8317

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