RE: [SI-LIST] : Trace Impedance Selection

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From: Ed Priest (Ed_Priest@pmc-sierra.com)
Date: Thu May 04 2000 - 14:16:51 PDT


Evan Davidson - IBM (E.E. Davidson) wrote a good article a few years back
looking at exactly this problem. It is a tradeoff between more current -
hence more SSN noise, or more tighly controlled fields - crosstalk effects,
etc. Some noise drive a high impedance and some push you to a lower one. His
conclusions when you summed them all up was that a range of 50-75 ohms give
you the best performance - about 65ohms being idea. Of course this answer
will vary slightly with your particular system noise situations.

One thing that no one has brought up is the RC charging time constant of the
load. The receiver will have a finite lumped load capacitance (ESD
structure, etc) and having a higher impedance will definitely increase the
charge time.

Thanks
Ed

-----Original Message-----
From: DORIN OPREA [mailto:doprea@newbridge.com]
Sent: Thursday, May 04, 2000 6:40 AM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Trace Impedance Selection

XTK simulation shows indeed almost the same signal at the receiver pin, but
a
different situation at the driver pin. A higher value of the trace impedance
determines higher overshoot at the driver pin.
This overshoot will generate crosstalk and EMI problems. Note that close to
the
driver pin is always a via to get the high speed signal on the internal
layers.
This via will amply the effect.

Have fun,

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