Transmission Line Effects - National Instruments PCI-6601 User Manual

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Transmission Line Effects

Note

Before using a voltage follower or series termination, use digital filtering to

eliminate measurement errors.

Table 3-8. Signals and D GND Pin Number on 68-Pin Connector Block (Continued)

PFI Number

PFI 36

PFI 37

PFI 38

PFI 39

Transmission line effects can degrade the signal and cause measurement

errors. Use twisted-pair wires to connect external signals to the device to

improve impedance matching and signal integrity. NI 660x devices provide

onboard series termination to reduce signal reflections when it drives an

output.

For reflection problems that occur when the device drives the signal,

use parallel AC termination at the destination. When using a National

Instruments SH68-68-D1 cable, the recommended values for R

68 Ω and 150 pF, respectively.

For reflection problems that occur when NI 660x devices receives the

signal, use series termination at the device driving the signal. The sum of

and the output impedance of the source is approximately 80 Ω.

Typically, this condition results in a value of approximately 50 Ω for R

To use serial termination when the source impedance is larger than 80 Ω,

use a voltage follower with low output impedance, and connect R

output of the voltage follower.

Pin Number for D GND

3-21

Chapter 3

Signal Connections

and C

at the

NI 660x User Manual

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