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

Mike Jenkins (jenkins@lsil.com)
Wed, 11 Feb 1998 11:48:42 -0800

Vinu Arumugham wrote:
>
> 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,

Yes, I agree -- less couplig gets more discontinuity. The one thing
your note doesn't say is the impedance of a single line over that gap.
That value is VERY large. The discontinuity for a dif'l pair is still
much less than for a single-ended line. (I calculate the glitch from a
50 mil split plane gap under that 100 ohm dif'l pair is equivalent to
the effect of a parasitic shunt capacitance of less than 0.1 pF (but
opposite polarity, of course).)

Regards,
Mike

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