NNTP-Posting-Date: Fri, 25 Oct 2002 11:43:31 -0500
From: "Dave VanHorn"
Subject: Re: Bypass capacitor position
Date: Fri, 25 Oct 2002 11:44:05 -0500
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> You have gotten some very good answers, but I haven't seen one
> detail. The pair of traces to the capacitor will produce a bit of I*R
> and L*(di/dt) drop during changes in the chip's current demand. As
> far as the stability of the power across the chip, it makes no
> difference how the total trace length is divided between the ground
> and power sides of the capacitor, so as long as the traces to the
> bypass capacitor share no other duties, it doesn't matter whether the
> cap is closer to ground or Vcc.
It does, if the cap's ground end dosen't have a low Z path directly to the
same chip's ground pin.
That's why I put mine at the chip's ground pin. If you think of the tracks
as small inductors, then the configuration is a tee filter. One inductor off
to the ps, and one off to the chip. You can't reduce the one between the
cap and the chip, but you can put it where it does the most good.
If you move the cap up to the vcc pin, then you have an inductor between the
cap and the ps, and an inductor between the ground end of the cap, and the
I also separate my VCC feeds as much as practical, and it's not uncommon for
me to have three or four VCC tracks meeting at the PS output cap. I also
watch my grounds, and section "A" that's fed by a discrete VCC track will
also have a discrete ground return. It's local ground will be planed to the
degree possible on both layers, but not contact any other section's ground
except at the PS output cap.
Two layer boards, plastic enclosures, no ferrites, foils, or sprays, and I
usually get the "is that thing on?" reaction at part 15.