Figure - National Instruments NI USB-621x User Manual

Chapter 4 Analog Input
AI Ground-Reference
Settings
RSE
NRSE
DIFF
NI USB-621x User Manual
The AI ground-reference setting determines how you should connect your
AI signals to the NI 621x device. Refer to Chapter 5,
on the USB-6210/6211
Ground-reference settings are programmed on a per-channel basis. For
example, you might configure the device to scan 12 channels—four
differentially-configured channels and eight single-ended channels.
NI 621x devices implement the different analog input ground-reference
settings by routing different signals to the NI-PGIA. The NI-PGIA is a
differential amplifier. That is, the NI-PGIA amplifies (or attenuates) the
difference in voltage between its two inputs. The NI-PGIA drives the ADC
with this amplified voltage. The amount of amplification (the gain), is
determined by the analog input range, as shown in Figure 4-2.
V
in+
V
in–
Table 4-3 shows how signals are routed to the NI-PGIA.
Table 4-3. Signals Routed to the NI-PGIA
Signals Routed to the Positive
Input of the NI-PGIA (V
AI <0..31>
AI <0..31>
AI <0..7>
AI <16..23>
For differential measurements, AI 0 and AI 8 are the positive and negative
inputs of differential analog input channel 0. For a complete list of signal
Devices, section for more information.
Instrumentation
Amplifier
PGIA
V = [V – V
m in+
Figure 4-2. NI-PGIA
Signals Routed to the Negative
) Input of the NI-PGIA (V
in+
AI GND
AI SENSE
AI <8..15>
AI <24..31>
4-4
Connecting AI Signals
+
Measured
V
m
Voltage
–
] × Gain
in–
)
in–
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