[SI-LIST] Re: Differential trace route question
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: "'doug@xxxxxxxxxx'" <doug@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Mon, 21 Oct 2002 16:12:45 -0700
I think the first option is to do nothing. For a pair separated by 20mils
center-to-center, the net length difference is about 30mils per turn ([pi *
trace_separation]/2). This equates to about 5pS skew. That may be
significantly less than the driver or other interconnect irregularities
(vias, wire bonds, etc.) are likely to introduce.
If you do compensate for the length difference...
You don't have to "lose control" of your differential impedance. You can
simply adjust the width of the traces (make them wider) while you decouple
them to adjust the inner trace's length. Of course, for the length that's
routed loosely-coupled, the advantages of a tightly-coupled differential
pair (reduction in EMI, reduced susceptibility to impedance discontinuities,
and common-mode noise rejection) are lost but, if you're only talking about
routing them loosely-coupled for a short distance, this drawback (and the
impedance discontinuity if you don't adjust your trace width to compensate)
is miniscule.
I believe you'll want to make the appropriate length adjustment during the
turn, so that there is no substantial time that the signals are traveling on
a tightly-coupled differential pair with a skew between them (during that
length, the impedance would not be what you expect). The exact method of
adding the length is irrelevant - the pair is not coupled at this point.
I believe making the adjustment at the corner is contrary to the constraints
you proposed, but it also accommodates a bi-directional bus. And it keeps
from adding pimples, which my daughters say should be avoided at all costs.
Jeff Loyer
-----Original Message-----
From: Doug Brooks [mailto:doug@xxxxxxxxxx]
Sent: Friday, October 18, 2002 4:52 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Differential trace route question
Hi all,
This question came up in a seminar and I'd be interested in your inputs.
Assume I have a differential pair extending "North" from a driver for a
ways. The pair turns "East" for a ways and then "South" for a ways before
terminating at a receiver. The inner trace is shorter than the outer trace.
I am interested in controlling skew, so I want to equalize the trace
length. I am also interested in controlling the differential impedance, so
I want to keep constant trace spacing. These requirements are mutually
exclusive! I will compromise what I have to on impedance to obtain equal
trace lengths.
I visualize that I have three options for increasing the length of the
inner trace.
1. I can "snake" the extra length of the inner trace with the constraint
that the added length must be added as close as possible to the receiver.
2. I can "snake" the extra length of the inner trace without that
constraint. That is, the snaking can take place anywhere that is convenient
along the trace.
3. I can add randomly placed little "pimples" along the inner trace (sort
of like __/\__ ) (pointing away from the outer trace) each one of which
would be small and insignificant but collectively they would add up to the
total length I need to add.
Does anyone have any strong opinions why any one of these is better than
any other, or why there is an even better alternative?
Doug Brooks
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