TLC8044 Datasheet, PDF (13/32 Pages) Texas Instruments – 12-BIT ANALOG-TO-DIGITAL INTERFACE FOR CHARGE-COUPLED DEVICE IMAGE SENSORS FOR SCANNERS

TLC8044
12-BIT ANALOG-TO-DIGITAL INTERFACE FOR
CHARGE-COUPLED DEVICE IMAGE SENSORS FOR SCANNERS
SLAS128 – JUNE 1997
PRINCIPLES OF OPERATION
CCD image sensor array output summary (continued)
Reset
Feedthrough
Reset
Level
Video
Level
Black
Amplitude
Period of
One Pixel
White
Period of
One Pixel
Figure 9. A Typical Charge-Coupled Display (CCD) Output Signal
CCD array analog-to-digital interface functions
The interface to the CCD array analog output and the conversion of the output into digital form involves the
following functions:
1. The video output waveform first has to be removed from the varying dc level on which it sits and shifted in
level to be compatible with an interface device running from a single 5-V supply rail.
2. Gain has to be applied to bring the signal up to the full-scale range of the analog-to-digital converter (ADC)
and a means provided to adjust static gain to compensate for variations between devices or multiple outputs
of color arrays. Once these static dc levels (offsets) and gain levels have been adjusted, dynamic
corrections are needed on a pixel-by-pixel basis.
3. Dynamic gain adjustment is needed to compensate for the fall off in output from the center to the ends of
the array when used in scanner applications (see Figure 10). Dynamic offset adjustments are required to
compensate for the pixel-by-pixel variation in black dc levels obtained from different CCD array elements.
4. DC restoration may optionally be required. Global adjustments of gain and offset across a whole scan are
respectively used to correct color balance and contrast and to change brightness.
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