[SI-LIST] : Fast edge termination choice

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From: Shayle Hirschman (shayle@mho.net)
Date: Fri Feb 25 2000 - 06:34:27 PST


SI experts:

I am interested in making a decision between series source termination and
parallel termination (probably something like 100 ohms to vcc and 100 ohms
to gnd) at the destination of a point to point route.

I want to use the fast slew rate option of the FPGA as the clock is 150 MHz
and there isn't much timing budget from chip to chip.

Does series source termination defeat the purpose of selecting the fast
slew rate option? Will it slow the edge transition time down, or will the
signal still transition and travel as fast as if there was no termination,
but simply be divided (thereby depending on the reflection at the
destination)?

I suspect the latter, except that I've heard people say that series source
termination reduces EMI by slowing the edges down. Could it be that it
reduces EMI, not by slowing the edges down, but rather just simply because
it terminates? And that, therefore, as far as EMI goes, parallel
termination would have a similar EMI result?

If the edge rate does not diminish by using series source termination, then
I would prefer that method since it uses only one resistor and has no DC
bias current as does parallel termination.

Any suggestions?

Thanks in advance.

Shayle

*************************************************************
Shayle I. Hirschman, Senior Engineer
Managing Director
Digital Design Solutions
http://www.digital-designs.com
shayle@mho.net
Phone 901/759-1802 Fax 901/759-2324

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