National Instruments DAQ M Series - Manual (Page 30)

Chapter 4 Analog Input
NI 6236 User Manual 4-6 ni.com
Figure 4-5. NI 6236 PGIA
Analog input ground-reference setting refers to the reference that the PGIA
measures against. Differential is the only ground-reference setting for
NI 6236 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
Connecting Analog Current Input Signals section for allowable 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.
Note On devices that allow multiple ground-reference settings, the settings are
programmed on a per-channel basis.
Table 4-2. Signals Routed to the NI-PGIA
AI Ground-Reference
Settings
Signals Routed to the Positive
Input of the NI-PGIA (V
in+
)
Signals Routed to the Negative
Input of the NI-PGIA (V
in–
)
DIFF AI <0..3>+ AI <0..3>–
RSE Internal Channels AI GND
I
in+
V
m
= I
in • R × Gain
V m I in–
PGIA +
–
Measured
Voltage
Instrumentation
Amplifier
Current
Sense
Resistor
AI GND