RE: [SI-LIST] : Diode Termination of Transmission Lines

Peterson, James F (jfpeterson@space.honeywell.com)
Wed, 10 Jun 1998 10:33:30 -0400

More on diode term's :

With a diode termination scheme and TTL level signals you'll settle
(eventually) around 3V but you can still overshoot up to 5.4V. So a
simple diode termination won't help you there (it would still help with
the undershoot noise).

Also, if you do have CMOS levels, couldn't you keep your clock trees
small (multiple drivers) and rely on the ESD protection diodes inside
the receivers to squash the under/over-shoot? This is one of the
approaches we use and it seems to be effective.

Take care,

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

----------
From: Wojciech_Babij@nmss.com [SMTP:Wojciech_Babij@nmss.com]
Sent: Wednesday, June 10, 1998 9:06 AM
To: pbaxter@cochlear.com.au
Cc: SI-LIST@silab.Eng.Sun.COM
Subject: Re: [SI-LIST] : Diode Termination of Transmission
Lines

My experience with Diode termination:

? Schottky Diode Termination
In some cases it can be expedient to use Schottky
diodes or
fast-switching silicon diodes to terminate lines. The
diode
switching time must be at least four times as fast as
the signal
rise time. Where line impedances are not well
defined, as in
breadboards and backplanes, the use of diode
termination is
convenient and can save time.
The Schottky diode's low forward voltage,
V(subscript: f)
(typically 0.3 to 0.45V), clamps the input signal to
a V
(subscript: f) below ground (lower diode) and
V(subscript: cc)+V
(subscript: f) (upper diode). This significantly
reduces signal
undershoot and overshoot. Some applications may not
require both
diodes.
? The advantages of diode termination are:
1. Impedance matched lines are not required
2. The diodes replace terminating resistors or RC termination
3. The diodes' clamping action reduces overshooting and
undershooting
4. Although diodes cost more than resistors, the total cost of
layout
might be less because a precise, controlled transmission-line
environment is not required
5. If ringing is discovered to be a problem during system
debug, the
diodes can be easily added.

Wojciech

pbaxter@cochlear.com.au on 06/10/98 06:09:39 AM

To: SI-LIST@silab.Eng.Sun.COM
cc: (bcc: Wojciech Babij)
Subject: [SI-LIST] : Diode Termination of Transmission Lines

Hi,

Might anyone be able to point me in the direction of some
PRACTICAL
application information on using Diode Termination on
Transmission
Lines. In particular for PCB1/Ribbon Cable/PCB2
interconnection as
well as one side of a PCB to the other side.

Most of the information that I have seen so far doesn't do
anything
more than give a brief overview. I was hoping on getting
information
on the diodes used and any of the pitfalls.

Thanks

Peter Baxter

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