Re: [[SI-LIST] : Trace Impedance Selection]

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From: Bill Owsley (wdowsley@netscape.net)
Date: Thu May 04 2000 - 10:40:41 PDT


Arbitary seems right; tho' I've tended to favor higher impedances for lower
current reasons, considering a lower enough impedance to get the load
capacitance charged in the time constraints. but I recently heard another
reason? for picking a certain value - in this case 50 ohms. This matches the
impedance of a small coax used to sample the signals for measurment. 8^)

Doug Brooks <doug@eskimo.com> 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
> .
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