A method of operating an analog-to-digital converter (ADC) includes providing the ADC including a plurality of stages, each including an operational amplifier, and a first capacitor and a second capacitor including a first input end and a second input end, respectively. Each of the first capacitor and the second capacitor includes an additional end connected to a same input of the operational amplifier. The method further includes performing a plurality of signal conversions. Each of the signal conversions includes, in an amplifying phase of one of the plurality of stages, applying a first voltage to the first input end of the one of the plurality of stages, randomly selecting a second voltage from two different voltages; and applying the second voltage to the second input end of the one of the plurality of stages.