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

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From: Vinu Arumugham (vinu@cisco.com)
Date: Fri Jul 28 2000 - 09:16:43 PDT


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
components.

Vinu

e wrote:

> Mike,
>
> 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.
>
> Ellis
>
>
> 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.
>>
>> Regards,
>>
>> Mark Gill, P.E.
>> EMC/Safety/NEBS Design
>> Nortel Networks - RTP, NC, USA
>>
>>
>> -----Original Message-----
>> From: Mayer, Mike [SMTP:mikem@artesyncp.com]
>> Sent: Tuesday, July 25, 2000 11:14 AM
>> To: 'si-list@silab.eng.sun.com'
>> 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
>> 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.
>>
>>
>> ===========================================================================
>>
>> =
>> Mike Mayer Artesyn Communication
>> Products, Inc
>> Senior Hardware Design Engineer Madison, WI
>> mikem@artesyncp.com http://www.artesyncp.com
>>
>> ===========================================================================
>>
>> =
>>
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