Apparatus for control of the range of input voltage values to be measured and for compensation of the changes in the supply voltage of an analog to digital dual slope converter circuit is described. The selection of a range for input voltage measurements in the analog to digital converter circuit permits the conversion to take place over a relatively long time period, thereby increasing the resolution of the resulting conversion. Changes in the supply voltage are combined with the input analog signal (during the capacitance charging period) and with the reference voltage (during the capacitance discharge cycle) providing compensation for the effects of time, temperature and the tolerance of the supply voltage.