An SAR ADC includes: at least one sub-ADC, configured to convert a corresponding input signal to a corresponding SAR code; and a tuning control unit, configured to adjust a full-scale voltage (VFS) of each of the sub-ADC to a corresponding predetermined target level in a VFS tuning mode. The tuning control unit generates a tuning code to control an adjusting capacitor array coupled to the sub-ADC for tuning the VFS. The tuning control unit controls the sub-ADC to convert plural reference voltages in the VFS tuning mode and extrapolating the conversion result to determine a corresponding calibrating VFS. The tuning control unit determines whether the calibrating VFS meeting the target VFS and loops the adjusting process in a linear search method or in a SAR method.