Re: [SI-LIST] : Trace Impedance Selection

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Wed May 03 2000 - 18:36:50 PDT


Comments interspersed below.

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?

I agree.

> 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!)

I've been in a situation where I could have chosen
a 50 ohm board or a 100 ohm board. I chose the
100 ohm construction. Why? Unproven but I took
100 ohm board as having 1/2 of the current draw
less than the 50 ohmer depending upon a lot of
things.

Now I have no idea if in fact my hunch was correct
but I wanted the lowest current that I could get
overall. And lower current to me meant lower emissions
(minus the contributions with edge rates of course).

1/2 current also translated to me as roughly 1/4
less power than the 50 ohmer. That translated to
lower thermals overall which was a big issue with
the board in question. Even if I wasn't even
near the 1/4 less power, I needed all the the
overhead for thermals as possible.

Again, I have no idea how in fact the same electronics
on a 50 ohm board would perform against a 100 ohm board.
I wasn't in the position to do an experiment like that.
Wish I had. Then I could sit here and say without a
doubt that 50 ohm boards do run hotter than 100 ohm
boards.

The emissions were quiet but that turned out to be
inconclusive at the time if I remember correctly. And
the thermals weren't such an issue as they were with
another 50 ohm board of similar construction (electronics
weren't one-to-one). And that was on the order of 7 to
10 degrees C. Significant with the product. No forced
air for cooling. So I did conclude that my choice did
have some effect. How much it makes in general, I
wouldn't know. - Doug

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