Re: [SI-LIST] : Trace Impedance Selection

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From: Scott McMorrow (
Date: Thu May 04 2000 - 11:02:19 PDT


Besides the possible EMI considerations thereare also timing
when choosing trace impedance.

When a trace is electrically long, it is
the trace Z0 which is charging receiver capacitance. The lower
the Z0
the faster the receiver charges and the faster the circuit timing.
This assumes,
that the driver is correctly matched to the line. With heavily
loaded circuits,
(for example, memory address lines) a lower Z0 can translate to
timing advantages.

A lower Z0 also shows smaller impedance perturbations to the
insertion of a capacitive
loaded receiver on a multidrop line.

A lower Z0 causes greater mismatch to long package stub sections
on multidrop
lines and can accentuate resonance effects at high edge rates.

A lower Z0 created by using wider traces shows less impedance
variation due
to board etching issues than a higher Z0 trace with exactly the
same dielectric

I lower Z0 trace will generally have lower coupling to neighboring

best regards,


Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231

Doug Brooks wrote:

> OK, gurus.... > > Lets say I have a trace on a board between two devices. There is nothing > that determines what the characteristic impedance of the trace should be. > That is, there are no device considerations, no space considerations, no > power considerations, etc. that would favor one value of characteristic > impedance over another. It is my opinion that as long as the characteristic > impedance is constant over the length and properly terminated, the choice > of impedance value in this situation is completely arbitrary. > > Does anyone (having accepted the assumptions above) have a different > opinion? In particular, are there any EMI reasons that would favor one > value of characteristic impedance over another? (But don't tell me the > assumptions above are wrong! That's not the question!) > > Doug Brooks > . > ************************************************************ > See our updated message re in-house seminars on our web page > . > Doug Brooks, President > UltraCAD Design, Inc. >

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