National Instruments DAQ X NI 634 Series User Manual page 58

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.
Figure 4-6. Differential Connections for Floating Signal Sources
Resistors
(see text)
+
Floating
Signal V s
Source
–
Bias
Current
Return
Paths
I/O Connector
Both inputs of the NI-PGIA require a DC path to ground in order for the NI-PGIA to work. If
the source is AC coupled (capacitively coupled), the NI-PGIA needs a resistor between the
positive input and AI GND. If the source has low-impedance, choose a resistor that is large
enough not to significantly load the source, but small enough not to produce significant input
offset voltage as a result of input bias current (typically 100 kΩ to 1 MΩ). In this case, connect
the negative input directly to AI GND. If the source has high output impedance, balance the
with Balanced Bias Resistors
AI+
Bias
AI–
Input Multiplexers
AI SENSE
AI GND
MIO X Series Device Configured in Differential Mode
X Series User Manual
Instrumentation
Amplifier
+
PGIA
+
Measured
–
V
m
–
© National Instruments | 4-15
Voltage