Chapter 5
Calibration
5-2
ni.com
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. It is better to self-calibrate when the device
is installed in the environment in which it is used.
Self-Calibration
The NI 6115/6120 can measure and correct for almost all of its
calibration-related errors without any external signal connections. NI-DAQ
software provides a self-calibration method. This self-calibration process,
which generally takes two to five 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 caused by
warming.
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, which is
discussed in the following section. If you are interested primarily in relative
measurements, you can ignore a small amount of gain error, and
self-calibration should be sufficient.
External Calibration
The NI 6115/6120 has an onboard calibration reference to ensure the
accuracy of self-calibration. Its specifications are listed in Appendix A,
. 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 are using the 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 the device.
An external calibration refers to calibrating the 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, so you should not have to perform
an external calibration very often. You can externally calibrate the device
by calling the NI-DAQ calibration function.
