Patent ID: 7772921

Claim:
A filter that filters an input signal to produce an output signal, comprising: a first compare voltage generation unit for generating a first compare voltage according to the input signal; a second compare voltage generation unit for generating a second compare voltage that equals a first reference voltage when the first compare voltage does not exceed the first reference voltage and equals a second reference voltage when the first compare voltage exceeds the first reference voltage; a comparator having a first input terminal and a second input terminal, coupled respectively to first and second voltage generation units for comparing the first compare voltage with the second compare voltage to produce at a comparator output a filtered signal according to the first compare voltage and the second compare voltage, wherein the second compare voltage generation unit includes a first resistor, a first switch connected in parallel with the first resistor and controlled by the filtered signal, a second resistor serially connected in with the first resistor, a current source, a second switch having an input switch terminal connected to the current source and an output switch terminal connected to the second input terminal of the comparator and grounded through the first and second resistors, the second switch controlled by the filtered signal or an inverse of the input signal so as selectively and electrically to connect the current source to the first resistor, wherein a current provided by the current source flows to the first resistor and the second resistor from the second switch to generate the first reference voltage, the current provided by the current source flows to the second resistor via the first switch to generate the second reference voltage, and a third switch having an input switch terminal connected to the comparator output and having an output switch terminal grounded, the third switch being controlled by the filtered signal or an inverse of the input signal so as selectively and electrically to connect the comparator output to a grounded terminal; and a first inverter, coupled to the comparator output for inverting the filtered signal to produce the output signal.