A filter system and filtering method includes a subtractor which receives an analog input signal and a reference voltage as a first input, and an analog feedback signal, supplied through a feedback loop, as a second input, and outputs a difference between the analog input signal and the analog feedback signal, and a low pass filter which outputs a digital signal by comparing the output signal of the subtractor and a reference voltage, then integrates duty cycle of the digital signal and calculates a following error amount, and then converts a low pass filtered signal based on the calculated following error amount to an analog signal in order to provide the analog signal to the analog feedback signal, i.e. the second input of the subtractor.