Re: [SI-LIST] : Ferrite bead

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From: Istvan NOVAK (istvan.novak@worldnet.att.net)
Date: Fri Mar 17 2000 - 05:00:26 PST


Sunil,

You can use ferrite beads on any plane wherever you dont have to carry
high-speed (return) current over the gap which is bridged by the bead.

In most single-ended systems ground (0V) serves as the primary return path,
so it usually should not be isolated by beads. . Power planes are OK to
isolate with ferrite beads as long as the corresponding ground plane is
solid under the high-speed traces. Also, isolated or 'floating' circuits,
such as some on-chip PLL/VCO subcircuits (with no need to carry signal over
the particular gap) are OK to isolate with ferrite beads. Similar but less
stringent rule applies to differential signalling as well. Similar, because
high ratio of common-mode/differential-mode impedance (which would mean
truly floating pair) is not practical in printed-circuit boards. Less
stringent, because the common-mode rejection of the circuit helps somewhat,
but common-mode rejection deteriorates at high frequencies, and the
common-mode voltage range of electronics may be very limited.

You should also remember to select a ferrite bead which will not resonate
with the surrounding circuit. This can be achieved by using lossy ferrites
(AC loss, not DC loss) and/or adding series resistors.

Istvan Novak

SUN Microsystems

----- Original Message -----
From: Sunil Kumar <sunilb@cdotb.ernet.in>
To: <si-list@silab.eng.sun.com>
Sent: Friday, March 17, 2000 3:19 AM
Subject: [SI-LIST] : Ferrite bead

>
> Can anybody tell me, why ferrite bead should not be used in power plane
> isolation in high speed environment and what are the options??
>
>
>
> Sunil Kumar Bahuguna
> Research Engineer
> ATM Group
> C-DOT
> Bangalore
> INDIA
>
>
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