Chapter 5
Calibration
6711/6713/6715 User Manual
5-2
www.ni.com
The loading factory calibration constants 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 will be used.
Self-Calibration
The 671X device can measure and correct for almost all of its
calibration-related errors without any external signal connections. Your
National Instruments software provides a self-calibration method. 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, gain, and linearity
drifts, particularly those due to warmup.
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 and you can ignore a small
1
amount of gain error
self-calibration should be sufficient.
External Calibration
The 671X device 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 are using 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
you can save the results in the EEPROM, so you should not have to perform
an external calibration very often. You can externally calibrate your device
by calling the NI-DAQ calibration function.
1
The onboard voltage reference has a temperature coefficient of 5 ppm/°C max (25 µV/°C). Therefore if the temperature
difference between the factory calibration and the service environment is less than 10 °C, the maximum gain error is less than
50 ppm, 0.005 percent at full scale output, after performing self-calibration.
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