1. A touch sensor, comprising:a sensor part including a first electrode and a second electrode spaced apart from each other;
a signal receiving part including a first terminal connected to the first electrode and a second terminal connected to the second electrode;
an amplifier circuit part connected between the second electrode and the second terminal;
an analog-to-digital converter part including a third terminal connected to another terminal of the signal receiving part and a fourth terminal connected to the second terminal and configured to output a digital signal corresponding to a voltage difference between the third and fourth terminals; and
a processor configured to detect a touch input from the sensor part in response to the digital signal when operating in a first mode, and to output a gain control signal for calibrating a gain value of the amplifier circuit part in response to the digital signal when operating in a second mode.