RE: [SI-LIST] : Ferrites on power leads

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From: David J. Anthony ([email protected])
Date: Mon Jul 31 2000 - 08:41:25 PDT

I have found a way to replace the ferrites, capacitors and inductors used
for filtering, with one component. In fact we have replaced as many as seven
components with one, while filtering over a much broader frequency range.
This is done with a circuit that is embodied in a single capacitive device.
This device is currently being implemented in production for motors in
automotive applications and is now being applied to boards in the
telecommunications industry. Test results of motor applications we have
worked on are at . If you go to the opening page, there
is a direct link to test results and articles that show the filter used on a
noisy motor.

Tony Anthony
X2Y Attenuators, LLC
(814) 835-8180
[email protected]
  -----Original Message-----
  From: [email protected]
[mailto:[email protected]]On Behalf Of Vinu Arumugham
  Sent: Friday, July 28, 2000 12:17 PM
  To: e
  Cc: Mark Gill; 'Mayer, Mike'; '[email protected]'
  Subject: Re: [SI-LIST] : Ferrites on power leads

  Why is a long narrow trace in series with a bead a poor filter? I expect
long narrow traces, being more inductive, to help block high frequency

  e wrote:

    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
        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
        devices with millions of transistors switching at multiples of the
        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]


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