A method of converting an analog input signal to a digital output signal includes adding a digitally controlled offset voltage into a comparison stage of a successive approximation analog-to-digital converter circuit, wherein the digitally controlled offset voltage has a periodic pattern including at least 2(K+1) steps, each of which has a value equal to an integer multiplying 2(−K) of an analog voltage corresponding to a least significant bit (LSB) of an N-bit digital signal, operating the successive approximation analog-to-digital converter circuit to sequentially generate at least a 2(K+1) number of N-bit digital signals based on the at least 2(K+1) steps of the digitally controlled offset voltage, summing the at least the 2(K+1) number of N-bit digital signals to obtain a summing result, and dividing the summing result through a divider block to obtain a digital signal having (N+K) bits.