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Chapter 5 Calibration
6023E/6024E/6025E User Manual 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 the device when
it is installed in the environment in which it will be used.
Self-Calibration
Your devicecan measure and correct for almost allof 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 a minute, 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. Ifyou are interested primarilyin relative
measurements, you can ignore a small amount of gain error, and
self-calibration should be sufficient.
External Calibration
Your device has an onboard calibration reference to ensure the accuracy of
self-calibration. Its specifications are listed in Appendix A, Specifications.
The referencevoltage is measuredat the factory and storedin theEEPROM
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 ofthe onboard reference is part of this process and
you cansave the results inthe EEPROM, soyou should not have toperform
an external calibration very often. 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. Use a reference that is several times more accurate than the
device itself.