Re: [SI-LIST] : AC Impedence / Buried Cap Question

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Thu Dec 07 2000 - 06:04:01 PST


Franz,

Without complex analysis I'd say, that the location matters on high freqs. A
bypass capacitor is a lumped element. However a plane pair as a cap is not
exactly lumped. A displacement current travelling through such a cap is
localized. Because of a size of capacitor. You can consider it to be a lumped
cap within a radius roughly 1/6 of your signal front's length in inches. (pretty
rough approximation).
So, if you have a VCC/GND bounce, because of a return current (through a plane,
I hope) and your lumped cap is 6 inches away, and your front is 1ns (6 inches),
and 1/6 of this is 1 inch - well, you don't have any good capacitance to smooth
it out except your 1-inch radius circle of a plane, which is definitely not
enough.

However, at low freqs the whole plane and all the caps on it are a lumped
capacitance, which acts as one within any reasonaby expected fron lengths, so
I'd say it does not matter.

Of course, you have to clarify what you call low freqencies.

Mike.

"Dill, Franz @ Celerity" wrote:

> All,
>
> A fellow co-worker mentioned: with buried capacitance the location of bypass
> capacitors (for that buried capacitance plane) does not matter (i.e. an IC's
> bypass cap. could be 6 inches away). Is this true?
>
> My Thoughts:
> At relatively low frequencies (say 5MHz) you cannot consider buried
> capacitance an AC short which would be required in allowing you to deem its
> impedence negligable. With a non-negligable BC impedence bypass caps cannot
> be as effective, say 6 inches away, from the IC. I forsee the potential of
> a low frequecy noise being injected on a power supply in such a bypassing
> configuration. Am I mistaken?
>
> Franz.
>
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