PURPOSE: To improve the quantization accuracy in a dark part without losing high speed performance of A/D conversion by providing a 2nd amplifier for amplifying a picture signal with a smaller amplification factor than that of a 1st amplifier on a reader.
CONSTITUTION: A picture output signal OS and a picture compensation output signal DOS are amplified by an operational amplifier 2 and inputted to a Vin terminal as an analog input signal to an A/D converter 4. Furthermore, the signals are amplified by an operational amplifier 3 whose amplification factor is smaller than that of the operational amplifier 2 and the amplified signal is inputted to a REF+ terminal as the upper limit reference input of the A/D converter 4. Since the gradient of a curve in the conversion characteristic of an output at A/D conversion to an input voltage at a dark part is steeper and slows down at a bright light since the amplification factor is increased by selecting the amplification factor of the operational amplifier 3 smaller than the amplification factor of the operational amplifier 2, the quantization accuracy of the dark part is improved.
