NNTP-Posting-Date: Sat, 26 Oct 2002 14:14:35 -0500
From: "Dave VanHorn"
Subject: Re: Bypass capacitor position
Date: Sat, 26 Oct 2002 14:15:07 -0500
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> So it's like audio: violent opinions all around but, in a blind test,
> everything sounds pretty much the same.
Except that my spectrum analyzer, and part 15 testing says "Zere EEs a
In one example, I was looking at the output of a switcher (flyback) and how
to handle the very simple output filter. Do I blob the diode, cap, and
output together, or make a "V" shaped track 100 mils wide to go from diode
to cap, and cap to output. CS tells me that the V should be better, and it
measured so, even though the leg of the triangle I cut to make it a "V" was
only 200 mils.
One thing that will make the whole thing pretty fuzzy, is bad grounding.
All the bypass caps in the world won't help you if you're running ground as
8 mil spaghetti.
Those murata 3 lead emi filters are a good example. I had a reputable
engineer, tell me that they really didn't make much difference. We set up
his board, and they really weren't working that well.. We then went over to
my board, and got the 50-60 dB filtering like the spec sheet says.
What's the difference? His filter ground lead was a thin track that
meandered a bit before hitting the uP ground pin whose supply he was
filtering, and there were other "customers" tapped into that track.