The invention discloses a successive-approximation-register analog-to-digital converter and method thereof. The converter comprises: a plurality of capacitors, a switch controlled by a sampling signal for connecting a common node to a ground node when the sampling signal is asserted; a plurality of switching networks controlled by the sampling signal and a plurality of control bits comprising a respective grounding bit and a respective data bit, each of the plurality of switching networks for connecting a bottom plate of a respective capacitor to an analog input signal, a ground node, a first reference voltage, or a second reference voltage depending on the asserted signal or bit; a comparator for detecting a polarity of a voltage at the common node and outputting a binary decision along with a complementary binary decision when a comparing signal is asserted; a logic gate for receiving the binary decision and the complementary binary decision and outputting a ready signal indicating whether a decision is readily made; a timer for receiving the comparing signal and outputting a time out signal; and a SAR logic for receiving the binary decision, the ready signal, and the time out signal and outputting the sampling signal, the comparing signal, the plurality of control bits, and an output data.