An app note from Atmel, voltage reference calibration and voltage ADC usage (PDF!):
• Voltage reference calibration.
– 1.100V +/-1mV (typical) and < 90ppm/°C drift from –10°C to +70°C.
• Interrupt controlled voltage ADC sampling.
• Calculation of voltage and temperature from voltage ADC measurements.
Some of the new Atmel AVR Smart Battery devices contain a very accurate low power bandgap voltage reference which when calibrated correctly has less than 90ppm/°C drift from –10°C to +70°C and absolute accuracy of typically +/-1mV.
This is the reference used for the internal Voltage ADC and Coulomb Counting ADC. ATmega16HVA and ATmega406 are examples of devices featuring this.
To achieve the high accuracy, the reference has to be calibrated according to the algorithm described in this application note. For some devices this calibration is done at the Atmel factory and the results are stored in the signature row. Calibration is then just a matter of reading out the data and storing it in relevant registers. Both methods are demonstrated in this application note.