National Instruments NI 6238 - Manual (Page 32)

Chapter 4 Analog Input
NI 6238/6239 User Manual 4-6 ni.com
Figure 4-5. NI 6238/6239 PGIA
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
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..7>+ AI <0..7>–
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