A method for updating a tap coefficient of a decision feedback equalizer is provided. The method includes sampling a first input signal received by a sampler of a decision feedback equalizer. It is determined if an amplitude of the first input signal falls within a range defined between a first predetermined voltage level and a second predetermined voltage level. If the amplitude of the first input signal falls outside the range, a tap coefficient is updated to generate an updated tap coefficient that is fed back to adjust an amplitude of a second input signal received at an input end of the decision feedback equalizer. If the amplitude of the first input signal falls within the range, the tap coefficient is free from being updated.