[SI-LIST] : Schottky diode termination

Peterson, James F (jfpeterson@space.honeywell.com)
Wed, 28 Oct 1998 07:12:13 -0500

John,
The clamp diodes inside CMOS IC's do help terminate, but at a cost - more
current through the power/ground pins (still, sometimes it worth it ;>).

Clamp diodes inside TTL devices are useless for termination on the high side
because they are so far above a TTL high signal (diode doesn't turn on until
~5.6V, while the signal is ringing around 3V).

Take care,
Jim Peterson
jfpeterson@space.honeywell.com <mailto:jfpeterson@space.honeywell.com>
Honeywell, Space Systems Division, M/S 934-5
13350 US 19 N., Clearwater, FL, 34624
813-539-2719

----------
From: Andrew Ingraham [SMTP:Andrew.Ingraham@digital.com]
Sent: Tuesday, October 27, 1998 6:39 PM
To: 'John.Philips@whitecross.com'
Cc: 'si-list'
Subject: RE: [SI-LIST] : Schottky diode termination

John,

I first saw Schottky diode termination described in a rather old
Fairchild ECL book. Diode termination was not often used for ECL,
perhaps because the normal ECL parallel termination usually worked
quite
well if you could afford the power dissipation.

The TTL logic families had clamp diodes in their input structures,
and
one of their intended functions actually was to provide some amount
of
diode termination. This is one of the reasons why TTL was easy to
work
with, sometimes even when the wires got long compared to the
falltimes.
Many people who have used TTL do not realize that they have been
making
use of diode termination all along.

Most CMOS families also have input clamp diodes, and again they
generally work as both input protection and partial signal
termination.

Those CMOS devices without input diodes tend to be SI nightmares.

In a multi-drop bus configuration, diode terminators may be needed
at
several places, perhaps as many as one per input. For a daisy-chain
route, they may only be needed at the two ends. The best way to
tell
what works is to try, preferably in simulation with GOOD models.

Regards,
Andy Ingraham

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