A/D conversion circuit

An A/D conversion circuit includes a reference voltage source to generate a calibration voltage, a multiplexer to receive an analog signal and the calibration voltage, and output the analog signal selected in a normal mode and the calibration voltage selected in a calibration mode or a self-diagnosis mode, an A/D converter to convert an output signal from the multiplexer into a digital signal, a non-volatile memory to hold the digital signal and calibration data, a digital calibration part to calibrate the digital signal in case of inputting the analog signal to the A/D converter in the normal mode based on the calibration data, and a self-diagnosis circuit to diagnose the A/D converter based on the digital signal in case of inputting the calibration voltage to the A/D converter in the self-diagnosis mode, and the digital signal stored in the non-volatile memory.