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Appendix D Hardware Considerations
© National Instruments Corporation D-7 653X User Manual
Using the Schottky-Diode Termination Scheme
You can terminate a cable that acts as a uniform transmission line in several
ways. If your 653X device is driving a cable, use the following termination
scheme: connect two Schottky diodes to each line, one to +5 VDC and the
other to ground. This termination will clamp any overshoot or undershoot
that occurs. The +5 V and ground connections should be low-impedance
connections. For example, if you make your +5 V connection through a
long wire, back to the +5 V pin of the 653X device, add a capacitor to your
termination circuit to stabilize the +5 V connection near the Schottky
diodes.
One suitable Schottky diode is the 1N5711, available from several
manufacturers. For more specialized use, you may be able to find diodes
packaged in higher densities appropriate to your application. For example,
the Central Semiconductor CMPSH-35 contains two diodes, suitable for
terminating one line. The California Micro Devices PDN001 contains
32 diodes, suitable for terminating 16 lines.
You do not need to add diodes to terminate the input signals. The
653X device contains onboard Schottky diode termination. Figure D-5
illustrates transmission line terminations.