Unexpected Value When Converting NI9775 Raw Binary Data to Voltage

Issue Details

In a cRIO system, I am using the NI‑9775. I am continuously acquiring data from the NI‑9775 at 1 MS/s and transferring that data to a host PC via Network Stream communication. In addition, I am saving the data transferred to the host PC.
For this reason, I am transferring raw binary data (I16) via Network Stream communication. In order to scale the raw binary data, I referred to the I/O properties shown at the link below:
https://www.ni.com/docs/ja-JP/bundle/ni-compactrio/page/ni-9775-fpga-interface.html

Using the “LSB Weight” and “Offset” values to calculate the scaled data using the following formula:
raw data × LSB Weight (nV) - Offset (µV) = scaled data

However, the result does not seem correct, and it is not working as expected. The calculated data is clearly different from the calibrated (scaled) data.
Is my calculation correct?

Solution

The NI9775 I/O Offset properties return I24 data type which not supported in LabVIEW. Since the data is use directly as I32 in LabVIEW, negative value of I24 will be converted to a very large value in I32 causing the conversion to produce incorrect result. As a workaround, I24 data need to be converted to I32 manually before using it to scale the raw binary data. 
Refer to below snippet on how to convert offset I24 data type to I32 data type:

Note: This is true as 2026Q1 version. The behavior might be different in future release once the FPGA I/O properties return a correct I32 data without additional conversion needed.

Additional Information

Below image showed the actual conversion result when using the returned Offset I/O properties directly. As the Offset value for AI0 and AI3 is very large, the conversion offset the result near -16V.

Converting the Offset value from I24 to I32 using the snippet above, the conversion produces the correct result as below:

For reference, below are the NI 9775 Calibration Constant Limits return by the LSB Weight and Offset properties. Without converting the I24 to I32 correctly, when the I24 return a negative value, it will be interpreted as a very large value in I32.

• LSB Weight (Gain): U24 in nV/LSB
  • Min: 1345000
  • Max: 1425000
• Offset: I24 in uV
  • Min: -352565
  • Max: 352565