For testing purposes, voltage reference was built of LM335-2.5 zener diode. But this is not always necessary. At certain conditions the cost of the voltage reference may be reduced down to simple resistor network. Calibration Supply two constant voltage sources to the input. Then rotate the trimmer resistor P1 and monitor the results until you get the constant offsets on both levels.
Offset value compensates internal systematic error and voltage reference error.
By default is set to 4. 4. The following two pictures represents test results using the AVR AT90S8515 running at 8 MHz. Number of samples per voltage for both graphs is 1.