Can I Change The Voltage Input Range of Analog Input Modules Using DAQmx Limits?

Issue Details

• In DAQmx, how are an Analog Input task's Maximum and Minimum values related to the NI Analog Input module input range?
• The input voltage of the NI-9215 is ±10 V but I just need to acquire a signal which goes from 0 to 10 V: is it possible to modify the input range to 0-10 V in order to take advantage of all the 16 bits of ADC resolution?

Solution

The "measurement resolution" (mainly referred to as Code Width) is based on the hardware range (also referred to as input range / device range) and the module's ADC resolution (bitness). The Code Width determines the smallest detectable change by the module. The formula is the following:

Code Width = Device Range/(2^bitness)

The DAQmx Input Limits are used to scale / clip the measurement using software as well as selecting the best device range (if multiple device ranges are available).

For example, the NI-9215 only has one available Input Range: ±10 V. Therefore, the DAQmx Maximum and Minimum values will not change the Code Width. Selecting [0, 10] as the Minimum - Maximum DAQmx range will result in the same Code Width as when selecting [-4, +4] or [-10, 10].

Since the NI-9215 Input Range ±10.0 V has a total span of 20 V, the Code Width will be: 20/(2^16) = 20/65536 = 305.17 μV.

Additional Information

As an example, we can compare the Specifications of the NI-9215 and PXIe-6363 in the image below. We can see that the PXIe-6363 has multiple available Input Ranges.

For the PXIe-6363, if you select Minimum and Maximum values of [0, 5] for your DAQmx virtual channel, DAQmx will automatically use the ± 5 V Device Range.

For additional information, refer to the Specifications Explained: C Series Modules page.