An analog-to-digital converter (ADC) for pipelined ADCs or cyclic ADCs is disclosed. The ADC includes at least one pair of two stages connected in series, and the two stages have different bits of resolution. An amplifier is shared by the pair of two stages such that the two stages operate in an interleaved manner. Accordingly, this stage-resolution scalable opamp-sharing technique is adaptable for pipelined ADC or cyclic ADC, which substantially reduces power consumption and increases operating speed.