[SI-LIST] : Bypass Cap via placement considered. Negative L's?

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From: Lawrence Butcher (Lawrence.Butcher@eng.sun.com)
Date: Mon Feb 26 2001 - 23:28:41 PST


A follow-up on my previous Bypass Cap via layout idea.

If it has any merit, I thought of a related idea.

You could place two same-valued caps side-to-side to try
to increase magnetic coupling, with the currents in the
two caps flowing in opposite directions.

   |---| |---|
   | |(+)- | Cap 1
   |---\ |---|
        (-) minimum safe spacing
   |---| \---|
   | -(+)| | Cap 2
   |---| |---|

Would this result in cancelling magnetic fields, reducing
the effective L of individual components?

Would this result in a lower ESR at and above the self-resonance
frequency for individual caps for the coupled pair than
you would get with the best possible via placement for
normally separated caps?

Once I know the answer about whether this idea is simply wrong,
I think that it will take calculations or measurements to find
out whether it actually results in better cap performance.

Lawrence

I don't know how far apart components can be to stay coupled.
Frequency-dependent, I assume. But caps don't work as bypass
devices to very high frequencies. Giga-Hzs are feet, so there
might still be hope.

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