National Instruments Data Acquisition Device NI USB-621x - Manual (Page 36)

Chapter 4 Analog Input
© National Instruments Corporation 4-3 NI USB-621x User Manual
For more information about setting ranges, refer to the NI-DAQmx Help or
the LabVIEW Help in version 8.0 or later.
The following table shows the input ranges and resolutions supported by
USB-621x devices.
Analog Input Ground-Reference Settings
USB-621x devices support the following analog input ground-reference
settings:
• Differential mode—In DIFF mode, USB-621x devices measure the
difference in voltage between two AI signals.
• Referenced single-ended mode—In RSE mode, USB-621x devices
measure the voltage of an AI signal relative to AI GND.
• Non-referenced single-ended mode—In NRSE mode, USB-621x
devices measure the voltage of an AI signal relative to the AI SENSE
input.
The AI ground-reference setting determines how you should connect your
AI signals to the USB-621x device. For more information, refer to one of
the following sections depending on your device:
• Connecting Analog Input Signals on USB-6210/6211/6212 Devices
• Connecting Analog Input Signals on USB-6215/6216/6218 Devices
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.
USB-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
Input Range
Nominal Resolution Assuming
5% Over Range
–10 V to 10 V 320
μV
–5 V to 5 V 160
μV
–1 V to 1 V 32
μV
–200 mV to 200 mV 6.4
μV