Successive approximation ad converter

A successive approximation analog-digital (AD) converter and method performed by the converter are provided. The successive approximation AD converter comprises a digital-analog (DA) converter; a comparator which determines a magnitude relation between an input signal and an output signal of the DA converter; and a successive approximation register which generates a first digital signal based on a determination result. The method comprises: switching an operation selection signal from a first logic to a second logic; performing a logical operation so that a digital signal input to the DA converter has a larger value or a smaller value than the first digital signal, when the operation selection signal has transited to the second logic, based on a portion of the determined first digital signal until transition; and inputting the first digital signal to the DA converter when the operation selection signal is the first logic.