Re: [SI-LIST] : Trace Impedance Selection

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From: Martyn Gaudion (ipc@polar.co.uk)
Date: Wed May 17 2000 - 13:38:58 PDT


Hi,

My understanding is that Rambus chose 28 Ohm to match the impedance
of the RDRAM...

Martyn Gaudion
www.polar.co.uk

At 12:40 05/05/00 -0500, you wrote:
>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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Kind Regards

Gwen Merchant - Marketing Support
Polar Instruments Ltd.
mail@polar.co.uk
Tel: +44 1481 253081
Fax: +44 1481 252476
www.polar.co.uk
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