Chapter5 Calibration
6023E/6024E/6025EUser Manual 5-2 ni.com
Thismethod of calibration is not very accurate because it does not take into
accountthe fact that the device measurement and output voltage errors can
varywith 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
Yourdevice can measure and correct for almost all of its calibration-related
errorswithout any external signal con nections. 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
assuringaccuracy in your application. Initiate self-calibration to minimize
theeffects of any offset, gain, and linearity drifts, particularly those due to
warmup.
Immediatelyafter self-calibration, the only significant residual calibration
error could be gain error due to time or temperature drift of the onboard
voltagereference. This error is addressed by external calibration, which is
discussedin 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
Yourdevice has an onboard calibration reference to ensure theaccuracy of
self-calibration.Its specificat ions are listed in Appendix A, Specifications.
Thereference 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 a re using your device at an extreme temperature or
ifthe onboard reference has not been measured for a year or more, youmay
wish to externally calibrateyour device.
An externalcal ibration refers to calibrating your device with a known
externalreference rather than relying on the onboard reference.
Redeterminingthe value of the onboard reference is part of this process and
youcan save the results in the EEPROM,so you should not have to perform
anexternal calibration very often. You can externally calibrate your device
bycalling the N I-DAQcalibration function.
Toexternally calibrate your device, be sure to use a very accurate external
reference. Use a reference that is severaltimes more accurate than the
deviceitself.