From: John Popelish
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Subject: Re: Bypass capacitor position
References: <3DB96152.1393B701@rica.net> <3DB9800A.780B223E@rica.net>
Date: Fri, 25 Oct 2002 22:43:00 GMT
NNTP-Posting-Date: Fri, 25 Oct 2002 18:43:00 EDT
Dave VanHorn wrote:
> John Popelish wrote:
> > Dave VanHoer wrote:
> > > It does, if the cap's ground end dosen't have a low Z path directly to
> > > same chip's ground pin.
> > > That's why I put mine at the chip's ground pin. If you think of the
> > > as small inductors, then the configuration is a tee filter.
> > I disagree, A tee filter passes a signal past a shunt path to
> > ground. In this case, there are two inductances in series with a
> > capacitance. The total impedance of the series string is not much
> > affected by how the total inductance is divided between the two ends.
> If the cap is mounted at the chip's ground pin, then the inductance between
> the cap's ground and the chip's ground has been minimized, and what
> inductance can be moved, has been moved to between the cap and the chip in
> the PS line. You'll never get it to ideal, but you can get it as close as
> possible to a tee filter.
But you still describe 3 reactances in series, not in a tee structure.
Here is a chip with a bypass capacitor and some trace inductance. How
is this a tee filter?
[c ] I
[h ] |
[i ] C
[p ] |
[ ] I
> > > Two layer boards, plastic enclosures, no ferrites, foils, or sprays, and
> > > usually get the "is that thing on?" reaction at part 15.
> > Do chips powered by one section communicate with chips powered by
> > another section?
> Yes, through narrow traces, and usually with 100 ohm resistors on or near
> the sourcing end.
You could reduce the noise between different sections if you gridded
the ground and 5 volt tree structure into a mesh work. And lower the
emissions some more.