transfer phase besides the switch 328 is used to connect the bottom plate of the capacitor 325 to the input common mode reference ground and the switch 329 is used to connect the top plate of the capacitor 325 to the output common mode reference ground the switch 334 is used to connect the bottom plate of the capacitor 331 to the input common mode reference ground and the switch 335 is used to connect the top plate of the capacitor 331 to the output common mode reference ground the multiplexer 323 is used to provide a first multiplexer output voltage selected from a group consisting of the negative reference voltage v refn the positive reference voltage v refp and the output common mode reference ground according to two select signals bp amp e and bn amp e wherein bp amp e is logic and of a positive bit signal bp and a clock signal e and bn amp e is logic and of a negative bit signal bn and the clock signal e and the rising edge of the clock e is the breaking point of the first half period and the second half period of sampling phase when bp amp e 0 and bn amp e 0 the first multiplexer output voltage is connected to the output common mode reference ground when bp amp e 0 and bn amp e 1 the first multiplexer output voltage is connected to the positive reference voltage v refp and when bp amp e 1 and bn amp e 0 the first multiplexer output voltage is connected to the negative reference voltage v refn therefore the first multiplexer output voltage is connected to the output common mode reference.