Single ramp comparison analog to digital converter

Abstract

An analog-to-digital converter of integration type comprises a charging and discharging capacitor, a first comparator for comparing an analog input voltage with the terminal voltage of the capacitor, a second comparator for comparing a reference voltage predetermined in relation to the input voltage with the terminal voltage of the capacitor, a switching transistor connected in parallel with the capacitor for controlling the charging and discharging of the capacitor, and a gated logic circuit applied with outputs of the first and second comparators, a charge-discharge control pulse, and a clock pulse. When the terminal voltage of the capacitor decreases below the reference voltage during discharge through the switching transistor, the discharge is stopped and a converted output is generated through the gated logic circuit on the basis of the outputs of the comparators. When the terminal voltage increases above the input voltage during charging, the converted output is stopped. The conversion speed and the conversion accuracy are improved and are not influenced by the dispersion of the elements in the discharging path. This converter is suitable for an exposure display system employing an IC for use in a camera. In such a case, the reference voltage may be selected at a value slightly lower than the voltage corresponding to the brightness of the scene on a rainy day.