An analog to digital converting apparatus and an initial method thereof are provided. The analog to digital converting apparatus includes a first and a second switching capacitor units, a circuit unit, a first and a second initialization switches, a third and a fourth capacitors and a logic buffer. The first and the second switching capacitor units respectively couple first capacitors and second capacitors to a first logic voltage, a second logic voltage or a first or a second input voltage according to a first control signal, and respectively generate a first and a second voltage. The circuit unit compares the first voltage and the second voltage to generate the first control signal. The first and the second initialization switches are respectively connected in series between the first and the second voltage and a common-mode endpoint. The logic buffer outputs the first or the second logic voltage to the common-mode endpoint.