Chapter 9 SCXI—Signal Conditioning

One-Point Calibration

Use one-point calibration when you need to adjust only the binary offset in your module. If you need to adjust both the binary offset and the gain error of your module, refer to the Two-Point Calibration section later in this chapter.

Note If you are using an E Series device, you should calibrate your DAQ device first using the E Series Calibrate VI.

Complete the following steps to perform a one-point calibration calculation in LabVIEW.

1.Make sure you set the SCXI gain to the gain you want to use in your application. If your modules have gain jumpers or DIP switches, set them appropriately. Refer to your SCXI module user manual

for jumper or switch setting information. If your modules have software-programmable gain, use the input limits parameter in the AI Config VI to set gain.

2.Program the module for a single-channel operation by using the

AI Config VI with the channel that you are calibrating as the channels parameter in the VI.

3.Ground your SCXI input channel to determine the binary zero offset. You should ground inputs because offset can vary at different voltage levels due to gain error. If you are using an SCXI-1100 or SCXI-1122, you can ground your input channels without external hookups by substituting the channel string with calgnd as the channel number. For other modules, you need to wire the positive and negative channel inputs together at the terminal block and wire them to the chassis ground.

4.Use the AI Single Scan VI to take several readings and average them for greater accuracy. Set the DAQ device gain settings to match the settings you plan to use in your application. By using the AI Start and AI Read VIs, instead of the AI Single Scan VI, you can average over an integral number of 60 or 50 Hz power line cycles (sine waves) to eliminate line noise. You now have your first volt/binary measurement: volt = 0.0 or the applied voltage at your input channel, and binary is your binary reading or binary average.

5.Use the SCXI Cal Constants VI with your volt/binary measurement from step 4 as the Volt/Amp/Hz 1 and Binary 1 inputs in your VI, respectively. (These input names may vary depending on your application setup.) For example, if your volt/binary measurement from step 4 was 0.00 V and 2, enter the values into your front panel controls.