2-2
|
ni.com
Chapter 2
Using the Module
Figure 2-1.
Connecting Floating Signal Source without Bias Resistors
For source impedances
≥
100
Ω
, this connection leaves the differential signal path significantly
off balance. Noise that couples onto the positive line does not couple onto the negative line
because it is connected to ground. This noise appears as a differential mode signal instead of a
common-mode signal, and therefore appears in your data. In this case, instead of directly
connecting the negative line to AIGND, connect the negative line to AIGND through a resistor
that is about 100 times the equivalent source impedance, as shown in Figure 2-2. The resistor
puts the signal path nearly in balance, so that about the same amount of noise couples onto both
connections, yielding better rejection of coupled noise. Since the bias resistor is between the
negative line of the floating source and AIGND, this configuration does not load down the
floating source output.
Figure 2-2.
Connecting Floating Signal Source with Single Bias Resistor
You can fully balance the signal path by connecting another resistor of the same value between
the positive input and AIGND, as shown in Figure 2-3. This fully balanced configuration offers
slightly better noise rejection, but has the disadvantage of loading the source down with the
series combination of the two resistors. For example if the source impedance is 2 k
Ω
and each
of the two resistors is 100 k
Ω
, the resistors load down the source with 200 k
Ω
and produce a -1%
gain error.
PXIe-4
3
02/4
3
0
3
TB-4
3
02
AI+
AI–
AIGND
+
–
Flo
a
ting
S
ign
a
l
S
o
u
rce
Low
Imped
a
nce
(<100
Ω
)
AI+
AI–
AIGND
PXIe-4
3
02/4
3
0
3
TB-4
3
02
AI+
AI–
AIGND
+
V
s
–
Flo
a
ting
S
ign
a
l
S
o
u
rce
R i
s
ab
o
u
t 100 time
s
s
o
u
rce imped
a
nce
AI+
AI–
AIGND
R
High
Imped
a
nce
(
≥
100
Ω
)
