From: e ([email protected])
Date: Thu Jul 27 2000 - 20:25:36 PDT
In my designs where I use ferrites for power supply filtering, I always
include caps on both sides of the ferrites. Also, the filtered side is
always a generously wide trace, if not a an isolated "island".
I agree with Mark that an improperly executed ferrite filter actually
increases noise. For example, a bead between a plane and a power pin
would probably not provide much benefit if the power pin were connected
to the bead with a long, narrow trace. In such cases, it would probably
be better to connect the power pin directly to the power plane and then
use a high frequency cap next to the pin to help filter noise.
The impedance you are adding is meant to block high frequency noise, not
the desired DC power levels. Generally this technique is useful for
noise-sensitive designs, like the front end of a receiver before A/D
conversion, even if planes are used. Like any other general
statements, there are likely to be exceptions, as Mark noted, specific
situations may require specific actions.
Mark Gill wrote:
> Mike -
> I have seen both effects, where it has filtered noise from loud
> power&ground planes, but also, where it increased the noise at the
> oscillator due to how the filtering was designed and implemented.
> Also, be careful about EMC recommendations - they are not generic in
> nature, and can lead to a great deal of both problems and costs if not
> applied to the correct situation. Specific situations lead to
> specific treatments.
> Mark Gill, P.E.
> EMC/Safety/NEBS Design
> Nortel Networks - RTP, NC, USA
> -----Original Message-----
> From: Mayer, Mike [SMTP:[email protected]]
> Sent: Tuesday, July 25, 2000 11:14 AM
> To: '[email protected]'
> Subject: [SI-LIST] : Ferrites on power leads
> 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
> In every other case we try to minimize the impedance of power
> 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.
> Mike Mayer Artesyn Communication
> Products, Inc
> Senior Hardware Design Engineer Madison, WI
> [email protected] http://www.artesyncp.com
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