[SI-LIST] : RE: Charge moving from decoupling capacitors

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From: Barry Ma (barry_ma@altavista.com)
Date: Thu May 18 2000 - 14:38:12 PDT


George,

Sorry, I forgot to repeat what I wrote to you 5/15/00:
"It is generally acknowledged that decaps and plane cap are complementary (supposing a 10 mil or less spacing between pwr and gnd planes). Decaps cover low end of frequency range, while the plane cap takes care of high frequencies."

I wrote similar words to Steve <weirsp@atdial.net> 5/17/00:
"If there's a 10 mil or less spacing between pwr and gnd planes, the plane cap is available. The plane cap and the decaps are complementary in whole frequency range. Plane cap takes care of high end, and decaps cover low portion. Then locations of decaps are not critical. And then decaps can be shared by other chips, according to the excellent research conducted by EMC lab at UMR."

That's my fault omitting this prerequisite today. In a PCB with 10 mil or less plane spacing, the plane capacitance would work better at 400 MHz than any decaps.

Please allow me to put some words about plane cap. Why do we - actually the EMC lab at UMR, insist on "10 mil or less"? Three reasons:
(1) Utilizable plane capacitor.
(2) Low inductance when pwr/gnd serve as a transmission line.
(3) Low mutual inductance between vias.

I apologize for not phrasing properly.

Thank you
Best Regards,
Barry
-----------
On Thu, 18 May 2000, George_Tang@Dell.com wrote:

> Barry,
>
> Thanks for the comments. Here are my comments:
>
> Ok, you put caps at a certain distance away from the IC because you only
> want them to work at 100 MHz. But that distance turns out to be the 1/4
> wave distance at 400 MHz, and you placed enough caps at the 1/4 wave
> distance to cause board resonance. Now what? Do you tell the caps not to
> work at 400 MHz because it's not their frequency?
>
>
> For your 2nd comment:
>
> I used the words "loosely define" for that reason. If you are interested in
> high frequency decoupling and instantaneous current, you really want to have
> all your charges moving in phase. At 1/4 wavelength, the charges are 90
> degrees out of phase, so they will not do much for your instantaneous
> current. 1/8 wavelength is what I consider to be acceptable. You can
> certainly pick a different number.
>
> Regards,
>
> George Tang
> george_tang@dell.com

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