Canon CANOSCAN D1230 series Service Manual page 37

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.
R
G
CCD
B
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 "65535" when input signal is
reference voltage. This converts 1 pixel signal into the image data of 65536 gradations for red,
green and blue each.
Blue Image Signal
Green Image Signal
Red Image Signal
Reference Voltage
2 - 14
COPYRIGHT
D/A Converter
Analog
Amplifier
Analog IC
Figure 2-14
A/D Converter
CCD_B
CCD_G
CCD_R
Vref
Output
+5V
Vcc
Figure 2-15
©
2001 CANON INC. CANOSCAN D1230U/D2400U REV.0 MAR. 2001 PRINTED IN JAPAN (IMPRIME AU JAPON)
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