[SI-LIST] : Ferrites on power leads

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From: Mayer, Mike (mikem@artesyncp.com)
Date: Tue Jul 25 2000 - 08:14:30 PDT


A couple of people here attended a seminar recently and were given advice on
a design. Among the suggestions were placing ferrite beads in series with
the power connection of all oscillators and in series with the power pins of
the board connector (it is a daughter card). I'm assuming the reasoning is
that this will "keep noise out of the power distribution system".

Is this technique valid for designs that use power and ground planes?

In every other case we try to minimize the impedance of power connections.
It seems counter-intuitive to take oscillators and try to add impedance to
their power connections. Are they really that much worse than modern CMOS
devices with millions of transistors switching at multiples of the bus
frequency and hundreds of sub-nanosecond I/O drivers? And does a board with
power and ground planes that plugs onto a board with power and ground planes
need ferrites in series with its power connections?

This seems like it improperly applying rules that may have applied to boards
without power and ground planes.

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Mike Mayer Artesyn Communication Products, Inc
Senior Hardware Design Engineer Madison, WI
mikem@artesyncp.com http://www.artesyncp.com
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