National Instruments Isolated Analog Input Device NI PXI-4224 - Manual (Page 48)

Chapter 5 Using the NI PXI-4224
NI PXI-4224 User Manual 5-12 ni.com
Calibrating the NI PXI-4224
Calibration refers to the process of minimizing measurement errors. On the
NI PXI-4224, 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-4224 to ensure
the accuracy of its analog circuitry. The first level, loading calibration
constants, is the fastest, easiest, and least accurate. The NI PXI-4224
automatically loads calibration constants stored in flash memory when
powered on. The intermediate level, internal calibration, is the preferred
method for assuring accuracy in your application. The last level, external
calibration, is the slowest, most difficult, and most accurate.
Loading Calibration Constants
The NI PXI-4224 is factory calibrated before shipment at approximately
23 °C to the levels indicated in Appendix A,
Specifications . The associated
calibration constants are stored in the onboard nonvolatile flash memory.
These constants are the values that were written to the CalDACs to achieve
calibration in the factory and the remaining signal conditioning error.
The digitizer calibration constants are automatically read from the flash
memory and loaded into the CalDACs by the NI PXI-4224 hardware the
next time the device driver software is loaded. The signal conditioning
calibration constants are also read from the flash memory at this time.
Self-Calibration
The NI PXI-4224 can measure and correct for most of its offset 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 for assuring accuracy in your application. Initiate an internal
calibration to minimize the effects of any offset drifts, particularly those
due to changes in temperature. To perform a self-calibration, complete the
following steps:
1. Double-click the Measurement & Automation Explorer icon on the
desktop.
2. Expand Devices and Interfaces to display the list of devices and
interfaces.
3. Expand NI-DAQmx Devices to display the list of NI-DAQmx devices.