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Chapter 4
Analog Input
Analog input ground-reference setting refers to the reference that the PGIA
measures against. Differential is the only ground-reference setting for NI
6238/6239 analog input signals, which means that the PGIA always
measures the voltages between AI + and AI – generated across the input
sense resistor, regardless of which analog input connection method is used.
Refer to the
input connection methods.
When measuring voltages internal to the device, such as the onboard
reference, the signal is measured against AI GND. This measurement
method is called referenced single-ended (RSE). The name is derived from
the fact that one end of the measurement, the positive input of the PGIA, is
connected to a signal, and the negative input of the PGIA is connected to
the AI GND reference.
Table 4-2 shows how signals are routed to the NI-PGIA.
AI Ground-Reference
Settings
DIFF
RSE
Note On devices that allow multiple ground-reference settings, the settings are
programmed on a per-channel basis.
NI 6238/6239 User Manual
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I
in+
Current
Sense
Resistor
I
in–
AI GND
Figure 4-5. NI 6238/6239 PGIA
Connecting Analog Current Input Signals
Table 4-2. Signals Routed to the NI-PGIA
Signals Routed to the Positive
Input of the NI-PGIA (V
in+
AI <0..7>+
Internal Channels
4-6
Instrumentation
Amplifier
+
PGIA
Measured
V
m
Voltage
–
R × Gain
V
= I
•
m
in
section for allowable
Signals Routed to the Negative
)
Input of the NI-PGIA (V
AI <0..7>–
AI GND
)
in–
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