National Instruments PCI-6281, User Manual

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Chapter 4 Analog Input
Figure 4-7. Differential Connections for AC Coupled Floating Sources with
Balanced Bias Resistors
Using Non-Referenced Single-Ended (NRSE)
Connections for Floating Signal Sources
It is important to connect the negative lead of a floating signals source to AI GND (either directly
or through a resistor). Otherwise the source may float out of the valid input range of the NI-PGIA
and the DAQ device returns erroneous data.
Note (NI USB-62xx BNC Devices) To measure a floating signal source on USB
BNC devices, move the switch under the BNC connector to the FS position.
Figure 4-8 shows a floating source connected to the DAQ device in NRSE mode.
Figure 4-8. NRSE Connections for Floating Signal Sources
All of the bias resistor configurations discussed in the
Using Differential Connections for
Floating Signal Sources
section apply to the NRSE bias resistors as well. Replace AI- with
AI SENSE in Figures 4-4, 4-5, 4-6, and 4-7 for configurations with zero to two bias resistors.
The noise rejection of NRSE mode is better than RSE mode because the AI SENSE connection
is made remotely near the source. However, the noise rejection of NRSE mode is worse than
DIFF mode because the AI SENSE connection is shared with all channels rather than being
cabled in a twisted pair with the AI+ signal.
Using the DAQ Assistant, you can configure the channels for RSE or NRSE input modes. Refer
to the
Configuring AI Ground-Reference Settings in Software
section for more information about
the DAQ Assistant.
–
+
AI GND
V
s
AC Co u pled
Flo a ting
S ign a l
S o u rce
AI+
AI–
AI S EN S E
AC Co u pling
M S erie s Device
–
+
AI GND
R
AI S EN S E
AI
V
s
Flo a ting
S ign a l
S o u rce
M S erie s Device