A signal calculator includes a capacitor and a variable current source. The variable current source charges the capacitor and generates a current corresponding to a predetermined voltage during a first period. A first voltage is generated using a voltage of the capacitor charged during an enable period of a first signal in the first period, and a second voltage is generated using a voltage of the capacitor charged during the first period. The variable current source further generates a current corresponding to the second voltage during a second period. A second signal is generated according to a result of comparing the first voltage with the voltage of the capacitor during the second period, and a fourth voltage is generated by generating and sampling a third voltage which is increased according to a first current during an enable period of the second signal in the second period.