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CHAPTER 2
2. A/D Conversion Output signal from the CCD is an analog signal which cannot be used as image data. So RGB output signal from the CCD is amplified by analog amplifier to generate analog data. The generated data is converted into averaged analog signal by D/A converter, then got feedback to the A/D converter to output constant digital data to the ASIC.
D/A Converter
CCD
R G B
Analog Amplifier
A/D Converter
Digital data To ASIC
Analog IC
Figure 2-13
1) D/A Converter D/A converter removes ununiform analog data generated by the CCD. It adjusts CCD output to keep max. 5V of input signal to the A/D converter, to make the black level of the image constant. 2) A/D Converter The A/D converter converts the black-level-corrected image signal (analog signal) to a 16-bit image data (digital signal) in the order of red, green and blue image signal. 5V is applied to the Vcc terminal and reference voltage is applied to the Vref terminal. A/D converter outputs �0� when input signal is 5V, and outputs �255� when input signal is reference voltage. This converts 1 pixel signal into the image data of 65536 gradations for red, green and blue each.
Image Data 01 01 01 01 01 01 +5V Vcc 01 01
A/D Converter Blue Image Signal Green Image Signal Red Image Signal Reference Voltage CCD_B CCD_G CCD_R Vref Output
Figure 2-14
COPYRIGHT  2002 CANON INC. CANOSCAN 8000F REV.0 OCT. 2002 PRINTED IN JAPAN (IMPRIME AU JAPON)
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