National Instruments Data Acquisition Device NI USB-621x - Manual (Page 61)

Chapter 4 Analog Input
NI USB-621x User Manual 4-28 ni.com
Figure 4-17. 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-6210/6211/6212 device, as shown in Figure 4-18. 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