Chapter 3
Using the NI PXI-4204
©
National Instruments Corporation
3-23
Calibrating the NI PXI-4204
Calibration refers to the process of minimizing measurement errors. On the
NI PXI-4204, errors from the digitizer components of the DAQ device
circuitry are corrected in the analog circuitry by onboard calibration
digital-to-analog converters (CalDACs). Errors from the signal
conditioning circuitry are corrected in software.
Three levels of calibration are available for the NI PXI-4204 to ensure
the accuracy of its analog circuitry. The first level, loading calibration
constants, is the fastest, easiest, and least accurate. The NI PXI-4204
automatically loads calibration constants stored in flash memory when
powered on. The intermediate level, internal calibration, is the preferred
method of assuring accuracy in your application. The last level, external
calibration, is the slowest, most difficult, and most accurate.
Loading Calibration Constants
The NI PXI-4204 is factory calibrated before shipment at approximately
25 °C to the levels indicated in Appendix A,
calibration constants (the values that were written to the CalDACs to
achieve calibration in the factory and the remaining signal conditioning
error) are stored in the onboard nonvolatile flash memory. The digitizer
calibration constants are automatically read from the flash memory and
loaded into the CalDACs by the NI PXI-4204 hardware the next time the
device is powered on. The signal conditioning calibration constants are also
read from the flash memory at this time.
Self-Calibration
The NI PXI-4204 can measure and correct for most of its
calibration-related errors without any external signal connections. This
calibration method is referred to as internal calibration or self-calibration.
This internal calibration process, which generally takes less than
two minutes, is the preferred method of assuring accuracy in your
application. Initiate an internal calibration to minimize the effects of any
offset and gain drifts, particularly those due to changes in temperature.
During the internal calibration process, the AI channels are compared to
the NI PXI-4204 onboard voltage reference. The majority of the offset
and gain errors in the analog circuitry are compensated for by adjusting the
CalDACs to minimize these errors. To perform a self-calibration, complete
the following steps:
1.
Double-click the
Measurement & Automation Explorer
icon on the
desktop.
