National Instruments PCI-6601, Руководство пользователя

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Chapter 3 Signal Connections
© National Instruments 3-21 NI 660x User Manual
Transmission Line Effects
Transmission line effects can degrade the signal and ca
u
se meas
u
rement
errors. Use twisted-pair wires to connect external signals to the device to
improve impedance matching and signal integrity. NI 660
x devices provide
onboard series termination to red
u
ce signal reflections when it drives an
o
u
tp
u
t.
For reflection problems that occ
u
r when the device drives the signal,
u
se parallel AC termination at the destination. When
u
sing a National
Instr
u
ments SH68-68-D1 cable, the recommended val
u
es for R
P
and C
P
are
68
Ω
and 150 pF, respectively.
For reflection problems that occ
u
r when NI 660 x devices receives the
signal,
u
se series termination at the device driving the signal. The s
u
m of
R
S
and the o
u
tp
u
t impedance of the so
u
rce is approximately 80
Ω
.
Typically, this condition res
u
lts in a val
u
e of approximately 50
Ω
for R
S
.
To
u
se serial termination when the so
u
rce impedance is larger than 80
Ω
,
u
se a voltage follower with low o
u
tp
u
t impedance, and connect R
S
at the
o
u
tp
u
t of the voltage follower.
Note Before
u
sing a voltage follower or series termination,
u
se digital filtering to
eliminate meas
u
rement errors.
PFI 36 39
PFI 37 41
PFI 38 36
PFI 39
Table 3-8. Signals and D GND Pin Number on 68-Pin Connector Block (Continued)
PFI Number Pin Number for D GND