An A/D converter system has an A/D converter element (10) which provides a first predetermined number of digital output bits for each input analog signal. An addition device (12,70) adds linear slope potential to the input analog signal. A calculator (14) provides an average of a plurality of digital output signals of the A/D converter element so that the average has larger number of bits than the first predetermined number. The addition device is implemented by a time constant circuit (70) which functions as an integral circuit for an input analog signal, and functions as a differential circuit for a slope potential.