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Chapter 5
Calibration
Self-Calibration
External Calibration
DAQCard-1200 User Manual
This method of calibration is not very accurate because it does not take into
account the fact that the device measurement and output voltage errors can
vary with time and temperature. Self-calibrate the device when it is
installed in the environment in which it is used.
The DAQCard-1200 can measure and correct for almost all of its
calibration-related errors without any external signal connections. NI-DAQ
provides a self-calibration function. This self-calibration process, which
generally takes less than two minutes, is the preferred method of assuring
accuracy in your application. Initiate self-calibration to minimize the
effects of any offset and gain drifts, particularly those due to warm-up.
Immediately after self-calibration, the only significant residual calibration
error could be gain error due to time or temperature drift of the onboard
voltage reference. This error is addressed by external calibration. If you are
interested primarily in relative measurements, you can ignore a small
amount of gain error, and self-calibration should be sufficient.
The DAQCard-1200 has an onboard calibration reference to ensure the
accuracy of self-calibration. Its specifications are listed in Appendix A,
Specifications. The reference voltage is measured at the factory and stored
in the EEPROM for subsequent self-calibrations. This voltage is stable
enough for most applications, but if you use your device at an extreme
temperature or if the onboard reference has not been measured for a year or
more, you may wish to externally calibrate your device.
An external calibration refers to calibrating your device with a known
external reference rather than relying on the onboard reference.
Redetermining the value of the onboard reference is part of this process and
the results can be saved in the EEPROM. You can externally calibrate your
device by calling the NI-DAQ calibration function.
To externally calibrate your device, be sure to use a very accurate external
reference. The reference should be 4–10 times more accurate than the
device itself.
5-2
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