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

Vinu Arumugham (
Thu, 2 Sep 1999 10:44:11 -0700


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.


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
> --
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Mike Jenkins Phone: 408.433.7901 _____
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