Chapter 4 Analog Input
© National Instruments Corporation 4-29 NI USB-621x User Manual
Figure 4-18. Differential Connections for Floating Signal Sources
with Balanced Bias Resistors
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
1MΩ). 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 4-19.
USB-6210/6211/6212 Device Configured in Differential Mode
PGIA
–
+
–
+
–
+
Floating
Signal
Source
Bias
Current
Return
Paths
AI GND
AI SENSE
Input Multiplexers
Measured
Voltage
Instrumentation
Amplifier
AI+
AI–
I/O Connector
Bias
Resistors
(see text)
V
s
V
m
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