Disclosed is a decoder circuit for a pulse amplitude modulation signal and a method of decoding a pulse amplitude modulation signal. The pulse amplitude modulation signal has a zeroth signal level, a first signal level, a second signal level and a third signal level. The decoder circuit comprises a first decision circuit, and a mapping circuit. The first decision circuit receives the pulse amplitude modulation signal and generates a low output signal for the first and the zeroth signal level, and generates a high output signal for the third and the second signal level. The mapping circuit receives the pulse amplitude modulation signal and generates a low output signal for the second and first signal level, and generates a high output signal for the third and zeroth signal level. Optionally, the decoder circuit comprises a logic circuit. The logic circuit receives the generated signal of the mapping circuit and the generated signal of the first decision circuit and generates a low output signal or a high output signal according to a predetermined truth table.