National Instruments DAQ X Series User Manual page 63

Bias
Resistors
(see text)
+
Floating
V
Signal
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 signal path as previously
described using the same value resistor on both the positive and negative
inputs; be aware that there is some gain error from loading down the source,
as shown in Figure 4-7.
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AI+
AI–
Input Multiplexers
AI SENSE
AI GND
MIO X Series Device Configured in Differential Mode
Figure 4-6. Differential Connections for Floating Signal Sources with
Balanced Bias Resistors
4-17
Chapter 4 Analog Input
Instrumentation
Amplifier
+
PGIA
+
Measured
–
V
Voltage
m
–
X Series User Manual