Differential Connections For Floating Signal Sources - National Instruments 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.
Figure 4-27. Differential Connection for Floating Signals on
Floating
+
V
Signal
s
–
Source
Bias
Resistors
Bias
Current
Return
Paths
I/O Connector
Note (NI USB-6346/6356/6366 BNC Devices)
source on X Series USB BNC devices, move the switch under the BNC connector to
the FS position.
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 1MΩ 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, which
saturates the instrumentation amplifier, causing erroneous readings. You must reference the
source to the respective channel ground.
DC-Coupled
You can connect low source impedance and high source impedance DC-coupled sources:
• Low Source Impedance—You must reference the source to AI GND. The easiest way to
make this reference is to connect the positive side of the signal to the positive input of the
instrumentation amplifier and connect the negative side of the signal to AI GND as well as
Simultaneous MIO X Series Devices
Simultaneous X Series Device
AI 0 +
AI 0 –
AI 0 GND
AI 0 Connections Shown
X Series User Manual
Instrumentation
Amplifier
+
+
V
–
m
–
To measure a floating signal
© National Instruments | 4-43
Measured
Voltage