Differential Connections For Floating Signal Sources - National Instruments DAQ X Series User Manual

Differential Connections for Floating Signal Sources

Figure 4-27 shows how to connect a floating (or non-referenced) signal
source to a channel on an Simultaneous MIO X Series device.
Floating
+
Signal
V
s
Source –
Bias
Resistors
Bias
Current
Return
Paths
I/O Connector
Figure 4-27 shows bias resistors connected between AI 0–, AI 0+, and the
floating signal source ground. These resistors provide a return path for the
bias current. A value of 10 kΩ to 100 kΩ is usually sufficient. If you do not
use the resistors and the source is truly floating, the source is not likely to
remain within the common-mode signal range of the instrumentation
amplifier, so the instrumentation amplifier saturates, causing erroneous
readings. You must reference the source to the respective channel ground.
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Simultaneous X Series Device
AI 0 +
AI 0 –
AI 0 GND
AI 0 Connections Shown
Figure 4-27. Differential Connection for Floating Signals on Simultaneous MIO
X Series Devices
4-49
Chapter 4 Analog Input
Instrumentation
Amplifier
+
+
Measured
V
–
m
Voltage
–
X Series User Manual