Re: [SI-LIST] : Q: Plane-jumping return currents

Mike Jenkins (jenkins@lsil.com)
Thu, 23 Sep 1999 13:24:07 -0700

Eric,

People may have made this point already, but in case not, here it
goes: Even though the return currents for a dif'l pair may travel
primarily on the ground plane, they are (roughly) equal and opposite.
When the pair cross a split ground plane, these currents have little
trouble finding each other and "closing the loop". That is, the
ground plane currents form a racetrack pattern under the pair over
that segment of ground plane.

It is true that the dif'l impedance may be considerably higher for
the duration of the gap. But since the gap is only tens of mils,
the effective discontinuity is minescule.

Doug,

I'm not an EMC expert, but let me stick my neck out a bit. As you
say, the common mode component of a dif'l pair signal hits a big
discontinuity at a plane split and this induces EMI problems.
However, on the far side of the split, the common mode is reduced.
As an example, if this signal subsequently went out on a cable,
would it be better to a) encounter the split with the resulting EMI
inside an enclosure, then launch onto the cable with lower common
mode, or to b) not have the plane split and launch into the cable
with higher common mode and the resulting higher shield current?
It seems to me that such a "common mode filter" might be valuable.

Regards,
Mike

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