A SAR ADC, used for converting an analog input into an N-bit digital output in a conversion phase, includes: three comparators, each two capacitor sub-arrays, coupled to the three comparators respectively, wherein the two capacitor sub-arrays are used for sampling the analog input and providing two inputs for the corresponding comparator; and an SAR logic, coupled to the three comparators and the three capacitor arrays, for, in each conversion sub-phase, coupling two selected capacitors of each capacitor sub-array to a set of determined reference levels, coupling two capacitors, which were selected in a preceding conversion sub-phase, of each capacitor sub-array to a set of adjusted reference levels obtained based on a set of data outputted from the three comparators in a preceding conversion sub-phase, and then generating two bits of the N-bit digital output by encoding a set of data outputted from the three comparators.