National Instruments NI USB-621x User Manual
Page 57
Chapter 5
Connecting AI Signals on the USB-6210/6211 Devices
© National Instruments Corporation
5-7
Figure 5-3. 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 k
Ω to
1 M
Ω). 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
M Series Device Configured in DIFF 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