Differential Connections For Nonreferenced Or Floating Signal Sources; Common-Mode Signal Rejection Considerations - National Instruments NI PCI-6110 User Manual

Chapter 4 Connecting Signals
+
Floating
V
Signal
s
Source
–
Bias
Current
Return
Paths
Bias
Resistor
(see text)
I/O Connector

Common-Mode Signal Rejection Considerations

NI PCI-6110/6111 User Manual
Differential Connections for Nonreferenced or
Floating Signal Sources
Figure 4-5 shows how to connect a floating signal source to a channel on
the NI PCI-6110/6111.
ACH0+
100pf
ACH0–
ACH0GND
ACH0 Connections Shown
Figure 4-5. Differential Input Connections for Nonreferenced Signals
Figure 4-5 shows a bias resistor connected between ACH0– and the
floating signal source ground. If you do not use the resistor and the source
is truly floating, the source often floats outside the common-mode signal
range of the PGIA, and the PGIA saturates, causing erroneous readings.
You must reference the source to the respective channel ground.
Figure 4-4 shows connections for signal sources that are already referenced
to some ground point with respect to the NI PCI-6110/6111. In theory,
the PGIA can reject any voltage caused by ground-potential differences
between the signal source and the device. In addition, with
pseudodifferential input connections, the PGIA can reject common-mode
noise pickup in the leads connecting the signal sources to the device.
1M
10nf
4-12
Instrumentation
Amplifier
+
PGIA
+
Measured
– V
m
Voltage
–
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