National Instruments NI USB-621x - Manual (Page 56)

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices
NI USB-621x User Manual 5-6 ni.com
Figure 5-2. Differential Connections for Floating Signal Sources with Single Bias
Resistor
You can fully balance the signal path by connecting another resistor of the
same value between the positive input and AI GND on the USB-621x
device, as shown in Figure 5-3. 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 k
Ω and each of the two resistors is
100 k
Ω, the resistors load down the source with 200 kΩ and produce a –1%
gain error.
–
+
R is about
100 times
source
impedance
of sensor
AI GND
R
V
s
Floating
Signal
Source
AI+
AI–
AI SENSE