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

Wojciech_Babij@nmss.com
Wed, 10 Jun 1998 09:06:17 -0400

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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