National Instruments AT E Series - Manual (Page 104)

Chapter 5 Calibrating the Device
AT E Series User Manual 5-2 ni.com
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 AT E Series device can measure and correct for almost all of its
calibration-related errors without any external signal connections.
The NI software provides a self-calibration method you can use. This
self-calibration process, which generally takes less than a minute, is the
preferred method of assuring accuracy in your application. You should
initiate self-calibration to ensure that the effects of any offset, gain, and
linearity drifts, particularly those due to warmup, are minimized.
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 External Calibration
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 AT E Series 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 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.
To externally calibrate the device, be sure to use a very accurate external
reference. The reference should be several times more accurate than the
device itself. For example, to calibrate a 12-bit device, the external
reference should be at least ±0.005% (±50 ppm) accurate. To calibrate