Programming instructions

Chapter 22 SCXI Calibration—Increasing Signal Measurement Precision
LabVIEW Data Acquisition Basics Manual 22-6
© National Instruments Corporation
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. If you are using an
AT-MIO-16F-5, AT-MIO-64F-5, or AT-MIO-16X, use the MIO
Configure VI to enable dithering, which makes your averaged data
more accurate. The dither mode is always enabled on E-series devices.
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 Hz 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 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 volts and 2, then you would enter the values into your
front panel controls as shown in the following illustration.
Two-Point Calibration
These steps show you how to perform a two-point calibration calculation
in LabVIEW. You should use two-point calibration when you need to
correct both the binary offset and the gain error in your SCXI module.
Note
If you are using an AT-MIO-16F-5, AT-MIO-64F-5, or AT-MIO-16X device or
an E-series device, you should calibrate your DAQ device first using either the
MIO Calibrate VI or E-Series Calibrate VI.
Follow steps 1 through 5 in the previous section,
One-Point Calibration
.
6. Now apply a known, stable, non-zero voltage to your input channel at
the terminal block. This input voltage should be close to the upper limit
of your input voltage range for the given gain setting. For example, if
your input voltage range is –5 to 5V, you would want to apply an input
voltage that is as close to 5 volts as possible, but not exceeding 5 volts.