A comparing device includes a first current generating circuit arranged to selectively generate a first current and a second current different from the first current, according to a first control signal. The comparing device also includes a comparing circuit having a common node coupled to the first current generating circuit for comparing a first input signal and a second input signal to generate an output signal according to the first current, the second current, and a second control signal. The second control signal and the first control signal are in-phase with each other.