Patent ID: 7061416

Claim:
A sigma-delta A/D converter, comprising: an A/D converter configured to output a digital signal; a signal-magnitude detecting circuit coupled to the output of said 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 said A/D converter and the output of said 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 said 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 said differential circuit and an input of said A/D converter, wherein said 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 said analog signal, and wherein said A/D converter includes an amplifier configured to amplify a signal input from said filter circuit, said amplifier configured to change a gain in response to the magnitude indicated by the digital signal.