A source driving device includes a locking module, a controlling module and a decoding module. The locking module executes a locking process selectively in a first band or a second band according to a band setting signal in order to lock a first clock signal synchronized with a first display signal. The controlling module is coupled to the locking module for comparing a control voltage with a reference voltage in the locking process and generates the band setting signal accordingly. The decoding module is coupled to the locking module for generating a decoded signal according to the first display signal and the first clock signal.