RE: [SI-LIST] : Trace Impedance Selection

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From: Robert Haller (rhaller@cereva.com)
Date: Thu May 04 2000 - 15:17:26 PDT


Doug,
        There have been a lot of good responses to this one. I feel compelled
to take a shot at the impedance question, that I have also asked myself.
Zo=SQRT(L/C) so in general I have found and used;
High Zo (lower C) for lower current drivers (CMOS), but increased crosstalk.
Low Zo (higher C) for higher driver currents (ECL), but increased di/dt.

Generally this tradeoff along with a compact module stackup, matched
impedance
in etch, cables and connector converges people to the 50 -> 75 ohm regions.

In the old ECL machines, 50 Ohms seemed to worked better.
While the CMOS machines, tended to trend towards 75 ohms.

Today's trends appears to approach somewhere in the middle. In high speed
design
there is still that matching requirement so a connector (i.e. a 50 ohm
connector)
can drive the end solution. In some cases I have seen/used multiple
impedances
(for mixed technologies) are on one board.

my 0.02 after dodging the ILOVEYOU virus.
bob haller

Cereva Networks
100 Locke Dr.
Marlboro, Ma. 01752
Work: 508-486-9660 X365
Fax: 508-486-9661
E-Mail: rhaller@cereva.com

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 doug@eskimo.com
> UltraCAD Design, Inc. http://www.ultracad.com
>

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