Chapter 4

Analog Input

calibration method improves absolute accuracy, but it increases the nominal resolution of input ranges by about 5% over what the formula shown above would indicate.

Choose an input range that matches the expected input range of your signal. A large input range can accommodate a large signal variation, but reduces the voltage resolution. Choosing a smaller input range improves the voltage resolution, but may result in the input signal going out of range.

For more information about setting ranges, refer to the NI-DAQmx Help or the LabVIEW 8.x Help.

Table 4-1shows the input ranges and resolutions supported by NI 621x devices.

Table 4-1.Input Ranges for NI 621x

 

Nominal Resolution Assuming

Input Range

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

 

 

Analog Input Ground-Reference Settings

 

NI 621x devices support the analog input ground-reference settings shown

 

in Table 4-2.

 

Table 4-2.Analog Input Ground-Reference Settings

 

 

AI Ground-Reference

 

Settings

Description

 

 

 

 

DIFF

In differential (DIFF) mode, NI 621x devices measure the difference in

 

voltage between two AI signals.

 

 

RSE

In referenced single-ended (RSE) mode, NI 621x devices measure the

 

voltage of an AI signal relative to AI GND.

 

 

NRSE

In non-referenced single-ended (NRSE) mode, NI 621x devices measure

 

the voltage of an AI signal relative to the AI SENSE input.

 

 

© National Instruments Corporation

4-3

NI USB-621x User Manual