a signal processing circuit outputs a digital word responsive to incident light , and includes an analog integrated circuit having a first input terminal receiving a first analog signal during a first active period of a first switching signal and a second input terminal receiving a time varying reference signal ; an inverter circuit inverting and amplifying an output of the analog integrated circuit responsive to an activated enable signal ; and an output circuit generating the digital word . during a second active period of the first switching signal , the first input terminal is coupled to a data line for receiving a second analog signal corresponding to image charges of an image input element . the enable signal is deactivated between end points of the first and second active periods of the first switching signal .