National Instruments PXI-6289 User Manual page 60

Chapter 4 Analog Input
Figure 4-4. Differential Connections for Floating Signal Sources
However, for larger source impedances, this connection leaves the DIFF signal path significantly
off balance. Noise that couples electrostatically onto the positive line does not couple onto the
negative line because it is connected to ground. This noise appears as a DIFF-mode signal
instead of a common-mode signal, and thus appears in your data. In this case, instead of directly
connecting the negative line to AI GND, connect the negative line to AI GND through a resistor
that is about 100 times the equivalent source impedance. 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 electrostatically coupled noise. This configuration does not load down the source
(other than the very high input impedance of the NI-PGIA).
Figure 4-5. Differential Connections for Floating Signal Sources
You can fully balance the signal path by connecting another resistor of the same value between
the positive input and AI GND, as shown in Figure 4-6. This fully balanced configuration offers
slightly better noise rejection, but has the disadvantage of loading the source down with the
series combination (sum) of the two resistors. If, for example, the source impedance is 2 kΩ and
each of the two resistors is 100 kΩ, the resistors load down the source with 200 kΩ and produce
a -1% gain error.
4-14 | ni.com
without Bias Resistors
Floating
+
Signal
V
s
Source
–
Impedance
<100 Ω
with Single Bias Resistor
Floating
+
Signal
V
s
Source
–
R is about
R
100 times
source
impedance
of sensor
M Series Device
AI+
AI–
AI SENSE
AI GND
M Series Device
AI+
AI–
AI SENSE
AI GND