A sigma-delta A/D converter includes an A/D converter configured to output a digital signal, a signal-magnitude detecting circuit coupled to the output of the A/D converter to output a control signal responsive to a magnitude indicated by the digital signal, a D/A converter coupled to the output of the A/D converter and the output of the signal-magnitude detecting circuit to output an analog signal having a signal level responsive to the digital signal and the control signal, a differential circuit coupled to an external analog input and the output of the D/A converter to output a differential between the external analog input and the analog signal, and a filter circuit to couple between the output of the differential circuit and an input of the A/D converter, wherein the D/A converter is configured to control, in response to the control signal, a capacitance of a capacitor that discharges following charging of electric charge to supply an electric current of the analog signal.