Chapter 3
Signal Connections
3-22
ni.com
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. The NI 660
x
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 the NI 660
x
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
36
Table 3-9.
Signals and D GND Pin Number on 68-Pin Connector Block (Continued)
PFI Number
Pin Number for D GND