Re: [SI-LIST] : Differential pairs and place splits

Vinu Arumugham (vinu@cisco.com)
Wed, 11 Feb 1998 10:08:38 -0800

Mike,

If we consider a differential pair implemented with two ~50 ohm microstrip
lines 10 mil apart, we get a differential impedance of ~90 ohm. If the traces
were 20 mil apart, the differential impedance is ~100 ohm.
If these two pairs were routed over a split, in that section ( if the split is
wide enough) the impedances would be ~120 and ~150 ohm respectively. The
discontinuity caused by the split increases as the coupling between the traces
decreases. In terms of return currents, what you described as the U-turn
becomes weaker with less coupling. In other words, the traces are behaving more
as single-ended lines and the split is therefore more disruptive.

May be we are saying the same thing. Less coupling between traces means more
discontinuity at the split.

Vinu

Mike Jenkins wrote:

> Vinu Arumugham wrote:
> >
> > It depends on the amount of coupling between the differential pairs. In
> > many implementations of differential pair traces, there is a tighter
> > coupling to the reference plane than between the traces. Since the bulk of
> > the return current would flow through the lowest impedance path ( in this
> > case the plane ), a split in the plane would remain a signal quality and
> > EMI concern.
> >
> > Vinu
> >
>
> Vinu,
>
> Even if the ground plane carried significant return current, these
> currents
> are equal and opposite for the pair. Encountering a split plane causes
> the
> return current for one line simply to do a U-turn, becoming the return
> current for the other line. On the other side of the split, the same
> thing
> happens. The discontinuity is still MUCH less than for a single-ended
> line.
>
> Mike
> --
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