Re: [SI-LIST] : Ferrite Beads

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Mon Nov 20 2000 - 06:26:59 PST


Even if you have differential signals: they are coupled to planes and the
disruption in planes will generate the same effect of voltage drop across
ferrites, though lesser one. So, you either make the residual run very, very
short (same often is done when connecting couples to Hi-speed BGA chips, where
the plane is fragmented, but the run is so short, it does not generate whole lot
of reflections), plus try to modify the pair coupling type (diff. impedance
created by sheer traces' coupling, not by each one coupling to a plane) - this
way there's no disruption in the return current.

Matter'o'fact, you can try to make the pair very tightly coupled along the whole
run. This is even better.

Mike.

RMELLISON@aol.com wrote:

> Stephen, I agree that you wouldn't want a ferrite in the ground path of a CCA
> if you had single ended signals interfacing to other cards through the
> backplane. Anytime a single ended signal is referenced to a segmented plane
> that is coupled with a ferrite, the return current generates a voltage across
> the ferrite. Considering the ferrite in it's most simplistic model, this
> would be expressed by e=Ldi/dt+ir. If single ended signals are used across
> the backplane, they should be referenced to the ground plane (with no
> ferrite) on the CCA, if possible. If there is no control of where they are
> referenced in the backplane, tight coupling of the power planes to ground
> planes through high frequency caps in the vicinity of the signals lines is
> necessary.
> Richard Ellison
> RMEllison@aol.com
>
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