The Differences Between Self-Calibration and External Calibration

Issue Details

What is the difference between self calibration and External Calibration for a DAQ device or a DMM?

Solution

Self-Calibration 
Self-calibration can be done without any external connections. This procedure involves routing a known internal reference voltage to all channels of the board.  The reference voltage is then read at a variety of gain settings and compared to the expected value.  This reference voltage is temperature protected and is meant to be used as a way to compensate for temperature changes.  As a result, the calibration coefficients stored in the EEPROM can be adjusted to account for drift in the analog circuitry of the gain amplifier or ADC. This procedure is particularly useful if the board’s temperature has changed significantly. Because the performance of components depends upon the operating temperature, self-calibration is able to compensate for larger changes in operating temperature. However, note that self-calibration is only as accurate as the accuracy of the on-board reference voltage. Because the reference voltage can drift slightly over time, periodic external calibration is still very important.

Self Calibration will vary in the amount of time it takes on different devices. The purpose of self calibration is the same for every device. 

External Calibration: 
External calibration is the most involved calibration procedure and it requires a highly precise voltage source. When an external calibration is performed, the EEPROM calibration constants are physically overwritten and new ones are applied. NI recommends that this procedure be performed every 1 – 2 years (depending on the board) when high accuracy is important, because an ADC can drift over time. In addition, external calibration is also important when a board is being used at a temperature that is significantly higher or lower than the temperature at which it was calibrated. In these circumstances, re-calibrating the board at the temperature at which it will be used improves the accuracy of the measurement.

Additional Information

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.