©
National Instruments Corporation
4-1
4
Calibration
This chapter discusses the calibration procedures for the NI 447
X
. The
NI 447
X
comes with a calibration certificate. The certificate contains a
unique tracking number linking the device to the NI corporate databases
where the traceability information is stored.
Calibration refers to the process of minimizing measurement and output
voltage errors by making small circuit adjustments. On the NI 447
X
devices, these adjustments are made to the digital data coming from the
ADCs. NI-DAQ includes calibration functions for performing all of the
steps in the calibration process. Some form of device calibration is required
for all but the most forgiving applications. If you do not calibrate the
device, the signals and measurements could have significant offset and gain
errors. The four levels of calibration available are described in this chapter.
The first level is the fastest, easiest, and least accurate, whereas the last
level is the slowest, most complex, and most accurate.
Loading Calibration Constants
The NI 447
X
is factory calibrated at approximately 25 °C to the levels
indicated in Appendix A,
. Before shipment, the associated
calibration constants—the values that were written to the calibration
circuitry to achieve calibration in the factory—are stored in the onboard
nonvolatile memory (EEPROM). Because the calibration circuits have no
memory, they do not retain calibration information when the device is
unpowered. Loading calibration constants refers to the process of loading
the calibration circuits with the values stored in the EEPROM. NI-DAQ
determines when this is necessary and does it automatically.
Self-Calibration
The NI 447
X
can measure and correct almost all of its calibration-related
errors without any external signal connections. NI-DAQ provides a
self-calibration method. Initiate self-calibration by calling the DSA
Calibrate VI or the
Calibrate_DSA
function. 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
