Disclosed is a ramp signal generator including a resistance circuit coupled to a first voltage terminal and an output terminal and adjusting, according to a plurality of control signals, a resistance value applied to the output terminal, a ramp signal being output through the output terminal, and a current circuit coupled to a second voltage terminal and the output terminal and for adjusting, according to a clock signal, a current level applied to the output terminal and a change rate of the current level, wherein the plurality of control signals are generated according to an analog gain having a multiple range, and wherein the clock signal has a first frequency when the analog gain has a first range within the multiple range and has a second frequency when the analog gain has a second range within the multiple range.