Canon CanoScan 9900F Service Manual page 31

2. A/D Conversion
Figure 2-13 is a block diagram that shows an internal processing of analog IC in the image
processing system.
Output signal s from the CCD are analog signals, and they cannot be used as image data.
So, RGB output signal from the CCD is amplified by analog amplifier to generate analog data.
The data is then converted into image data by A/D converter.
R
G
CCD
B
1) D/A Converter
D/A converter removes uneven data generated by the CCD' analog data. D/A converter
adjusts CCD output to maintain a maximum value of input signal to A/D converter at 5V so
that the black level of the image be constant.
2) A/D Converter
A/D converter converts the black-level-corrected image signal (analog signal) into a 16-bit
image data (digital signal) in the order of red, green and blue image signal.
CanoScan 9900F uses 8-bit A/D converter. 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 equals to reference voltage. 1 pixel signal is converted
into image data with 65536 tone for each colors including red, green and blue.
Blue Image Signal
Green Image Signal
Red Image Signal
Reference Voltage
COPYRIGHT 2003 CANON INC. CANOSCAN 9900F REV.0 FEB. 2003 PRINTED IN JAPAN (IMPRIME AU JAPON)
D/A Converter
Analog
Amplifier
Analog IC
Figure 2-13
A/D Converter
CCD_B
CCD_G
CCD_R
Vref
Output
+5V
Vcc
Figure 2-14
Digital data
A/D Converter
To ASIC
Image Data
0 1
0 1
0 1
0 1
0 1
0 1
0 1
0 1
CHAPTER 2
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