RE: [SI-LIST] : Trace Impedance Selection-- 28 on Rambus

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From: Harris, George (George.Harris@compaq.com)
Date: Fri May 05 2000 - 05:51:12 PDT


Why 28 ohms on Rambus?
Because if you add multiple 2.2pf loads to a 55-75 ohm trace, the loaded
impedance will be about 28 ohms. To minimize reflections when the wave
transitions from the unloaded section of etch to the congested area where
all the loads are, you need the unloaded etch to have the same impedance, 28
ohms.

-George
George Harris
Tel: 508-467-8893
george.harris@compaq.com
COMPAQ Computer Corporation

-----Original Message-----
From: opamps cybernetics [mailto:opamps@hotmail.com]
Sent: Friday, May 05, 2000 1:41 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Trace Impedance Selection

Hi Gurus,

One thing for sure is that larger characteristic impedance means thinnner
trace on PCB which also means more fringing electric and magnetic fields. So

more emission and susceptibility. Definitely space is something to be
concerned.

So what value of impedance you choose is determined by the above and the
fact that the smaller characteristics impedance (thicker traces) require
powerful drivers. So power is the an issue here.

I do not know why Rambus choose the magic number of 28 Ohm. Does any body
know why 28 Ohm in Rambus?

Subas

>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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