An A/D conversion circuit converts an asymmetric analogue signal into a digital signal. In an A/D conversion circuit of a parallel type, a central value of a reference voltage is adjusted by means of an average of an input analogue signal, such as a reproduced RF signal from an optical disk, or the like. The average value of the input analogue signal is detected by means of a low-pass filter (LPF). Respective comparators compare the input analogue signal with the reference voltage, and encodes a result of comparison by means of an encoder. An A/D conversion characteristic determined by voltage values from a maximum value Vref1 to an average value and another A/D conversion characteristic determined by voltage values from the average value to a minimum value Vref2 become nonlinear, thereby becoming able to address asymmetry of the input analogue signal.

 
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> Illuminance sensor determining the duty ratio of a PWM signal based on a digital output of an A/D converter and light control apparatus

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