Re: [SI-LIST] : Schottky diode termination
Mon, 26 Oct 1998 15:43:30 -0500

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Dear all

Does anybody have experience of in using schottky diode terminations in
transmission line applications.


WhiteCross Systems Limited Tel +44 1344 300770
3A Waterside Park Fax +44 1344 301424
Cookham Road
RG12 1RB

*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.

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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 undershoo=
4.) Although diodes cost more than resistors, the total cost of la=
might be less because a precise, controlled transmission-line environme=
is not required
5.) If ringing is discovered to be a problem during system debug, =
diodes can be easily added.



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