The AI ground-referencesetting determines how you should connect your AI signals to the NI 621x device. Refer to Chapter 5, Connecting AI Signals on the USB-6210/6211 Devices, section for more information.
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.
Instrumentation
Amplifier
Vin+ | | PGIA | | | + |
| | |
| | | |
| | |
Vin– | | | | | Vm | Measured |
| |
| Voltage |
| | | | | | |
| | | | | | | – |
| | | | | | |
| | Vm = [Vin+ – Vin–] ⋅ Gain | |
Figure 4-2. NI-PGIA
Table 4-3shows how signals are routed to the NI-PGIA.
Table 4-3.Signals Routed to the NI-PGIA
AI Ground-Reference | Signals Routed to the Positive | Signals Routed to the Negative |
Settings | Input of the NI-PGIA (Vin+) | Input of the NI-PGIA (Vin–) |
| | |
RSE | AI <0..31> | AI GND |
| | |
NRSE | AI <0..31> | AI SENSE |
| | |
DIFF | AI <0..7> | AI <8..15> |
| | |
| AI <16..23> | AI <24..31> |
| | |
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