TLC8044 Datasheet, PDF (14/32 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 array analog-to-digital interface functions (continued)
Ideal Output
(As Corrected By Pixel
Shading Adjust Block)
Scanning System
Output Error
Pixel Width
Scanner System Pixel Outputs
Time
Figure 10. Scanner System Relative CCD Pixel Output
CCD scanner analog-to-digital interface subsystem
input dc level shift, output offset, and channel gain
The TLC8044 uses external operational amplifiers configured as differential amplifiers to remove the dc level
present in the CCD outputs by using the common mode voltages from the OS and DOS outputs for each channel
(see the functional block and system diagrams). DC bias is provided for the external differential amplifier from
the TLC8044 DAC output as shown in the system diagram in Figure 8. Without any residual offset from the CCD,
the differential amplifier minimum output is (DAC result)/2 and is uneffected by the external differential amplifier
gain setting (G). The offset at the output of the external differential amplifiers, including residual offset from the
CCD, should be low enough to ensure the CCD amplified signal is within the input common mode range of the
TLC8044 and that the offset can be adjusted out by the TLC8044 internal DACs.
The external differential amplifiers also provide the system gain for each channel to ensure the output amplitude
of each channel is greater than one half the ADC full-scale range. Variations between the RGB channels of the
CCD can have a 10 to 1 ratio in output. To minimize the offset at the amplifier output with the highest gain, the
external amplifiers should be configured for gains in the range 1/3 to 3 rather than 1 to 10 to compensate for
this output variation. This is achieved by scaling the gain setting resistors shown in the system diagram by the
gain factor (G) over this 1/3 to 3 range.
RGB channel multiplexer and sampler
For color CCD image sensor arrays, a combined three-input multiplexer and sampler is used enabling the use
of a single fast 12-bit ADC and DSP channel.
The TLC8044 multiplexer has three differential inputs for each of the RGB channel outputs and a further internal
input for each channel which is used to compensate for the residual offset in the input signal. This internal offset
compensation is provided by the TLC8044 three 8-bit plus sign DACs, which provide bipolar offset correction
with respect to the input reference levels. The DACs are updated through the serial interface.
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