Charge transfer circuit with leakage current compensating means

Abstract

A monolithic analog to digital converter having leakage current compensating circuitry is disclosed. First and second charge storage capacitors are formed in a single semi-conductor substrate. An analog signal to be converted and a reference signal are simultaneously applied to the first and second capacitors, respectively. Thereafter, a charge transfer circuit supplies the first capacitor with a plurality of discrete charge packets. The polarity of the charge packets is chosen so as to increase or decrease the voltage across the first capacitor in the direction of the voltage across the second capacitor. When the voltage across the two capacitors become equal, a differential comparator generates an output signal indicating the end of a conversion operation. The parameters of the first and second capacitors are chosen such that the amount of charge added to or subtracted from the second capacitor due to a thermally induced leakage current in the substrate is substantially equal to the amount of charge added to or subtracted from the first capacitor as the result of the thermally induced leakage current.