A digital apparatus includes a timing control circuit, a period counter, a phase digitizer and a calculation circuit. The timing control circuit generates a first control signal according to a square wave signal and a predetermined value. The period counter generates a first digital value according to a reference clock signal and the first control signal. The phase digitizer generates a second digital value according to a phase difference between the square wave signal and the reference clock signal. The calculation circuit generates an output digital value according to the first digital value and the second digital value. An object of obtaining a high-resolution digitization with a reasonable sampling clock is realized by effectively combining the period counter with the phase digitizer.