[SI-LIST] Re: Terminating a branched clock line

  • From: Doug Brooks <doug@xxxxxxxxxx>
  • To: kevinlfitzgerald@xxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Mon, 15 Apr 2002 17:17:39 -0700

IMHO  If a single series resistor is used, it MUST be as close as possible 
to the driver. After all, it is it plus the output impedance of the driver 
that terminates the line. Otherwise, there will be an impedance mismatch 
between in the line between the resistor and the driver.

Then the branch MUST be very close to the receiver(s). Otherwise, there 
will be an impedance mismatch at the branch point that will reflect 
everywhere and won't be handled well by the series termination. In 
particular, if the branch is very close to the driver, then the reflection 
back from one branch will see the properly terminated driver in parallel 
with the other branch and proper termination of the first branch cannot result.

Doug Brooks

At 03:11 PM 4/12/2002 +0000, kevinlfitzgerald wrote:

>I have a clock signal with a 1ns rise time which is distributed to 2
>parts.  The 50 Ohm lines are matched length within +/-50 thou after
>the branch.  Our concern lies with the source termination. Should the
>source termination be before the branch, or immediately after the
>branch ?
>Any thoughts,

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