Transmission Line Effects - National Instruments NI 660 Series User Manual

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

Transmission Line Effects

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
R
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.
Note Before using a voltage follower or series termination, use digital filtering to
eliminate measurement errors.
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PFI Number
PFI 36
PFI 37
PFI 38
PFI 39
and the output impedance of the source is approximately 80 Ω.
S
3-21
Chapter 3 Signal Connections
Pin Number for D GND
39
41
36
and C are
P P
S
at the
S
NI 660x User Manual
.