NNTP-Posting-Date: Fri, 25 Oct 2002 12:44:22 -0500
From: "Dave VanHorn"
References: <3DB96152.1393B701@rica.net> <3DB9800A.780B223E@rica.net>
Subject: Re: Bypass capacitor position
Date: Fri, 25 Oct 2002 12:44:53 -0500
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> > 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.
> > 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.