This paper proposes a new calibration technique for capacitor array SAR ADC, here we focus on the true value and the true weight of each capacitor. By measuring the capacitor, we calculate the true weight (Wi) of each weighted-capacitor, and map this weight into m-bit code Ci (m>n). With the normal conversion code Di (n-bit), we define the first n-bit of as the ultimate output code. The simulation indicates that this method can greatly suppress harmonics, and the ENOB gets improved greatly.