EEPROM: A read-only memory whose contents can be erased and reprogrammed using a pulsed voltage.
Calibration is important because the performance of any given analog-to-digital converter can vary according to temperature, input voltage, time, and other factors. Because of this, calibrating a DMM or data acquisition device must take these two factors into consideration.
When a DMM or DAQ card is calibrated, specific calibration constants are stored on the board’s EEPROM memory. These constants are used by the driver software, (DAQmx, NI-DMM, etc.) to return the appropriate value for a given measured voltage. This technique is absolutely essential, because no analog-to-digital converter (ADC) is capable of producing a perfectly linear response. In addition, the calibration constants can be adjusted through external calibration.
Self-calibration does not provide NIST certificates as external calibration does. NIST Traceability Certificate for my Device
A self-calibration is recommended to be done between the external calibration date and the end of the calibration period. After the calibration period there's no guarantee that the reference voltage that is used in self-calibration is within specifications.