Delta-sigma a/d

an anti-aliasing means for preventing an aliasing caused during sampling a received analog signal; an A-D modulating means for converting the analog signal to a digital signal by oversampling; a decimation means for decreasing the sampling frequency output from the A-D modulated signal and removing a noise in the high frequency region; a clock generating means for generating a clock signal in oder to adjust the operational time of each element of the converter; a reference voltage generating means, the converter further includes a reference voltage controlling means for changing the reference voltage generated from the reference voltage generating means to a voltage corresponding to a desired gain value.