National Instruments USB-621 Series User Manual page 66

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