A circuit and a calibrating method are provided. A pixel sensor senses a terminal voltage of a driving transistor during a sensing period. A calibration sensor senses a first predetermined voltage and a second predetermined voltage during a calibration period. An amplifying circuit amplifies the terminal voltage according to a gain, and amplifies the first predetermined voltage and the second predetermined voltage according to the gain. An analog to digital converter converts the amplified terminal voltage into a digital code, and converts the amplified first predetermined voltage into a first digital code and converts the amplified second predetermined voltage into a second digital code. A gain adjusting circuit adjusts the gain according to the first digital code and the second digital code. Accordingly, the gain of the amplifying circuit is calibrated.
