TLC8044 Datasheet, PDF (21/32 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
pixel offset compensation
The output of the dc restore circuit is passed to an adder which performs pixel-by-pixel offset compensation.
Compensation values can either be supplied externally at the multiplexed pixel rate, allowing different correction
values for each pixel in the array, or supplied from internal default values programmed through the serial bus.
Selection between the two sources is controlled through the serial bus. Two sets of internal default registers
are provided to allow correction values to be stored internally for use on even and odd pixels with selection
between the two sets under control of the ONE terminal (ONE low for even registers, ONE high for odd
registers). This feature allows correction of differing dc offsets output on even and odd pixels, which occur in
some CCD sensors, using internally stored data.
Pixel offset correction values are input or stored as 12-bit 2s complement numbers. Programmable internal
scaling is provided which allows the offset correction factors to cover ±0.5, ±0.25, ±0.125, or ±0.0625 of the ADC
full-scale range. The internal pixel correction registers are reset to zero.
pixel shading compensation
This stage is implemented as a digital multiplier which corrects for nonuniform shading using externally supplied
12-bit unsigned values. The external correction factors are supplied at the multiplexed pixel rate. The external
correction range is from 0 to 4, which allows shading nonuniformity of up to 75% (i.e., the minimum input signal
is 25% of the peak) to be corrected without loss of resolution in the high gain pixels at the center of the scan.
Internal default registers are provided to set the gain through this block during calibration. The internal registers
default to a value of 1 (equivalent to decimal 1024 in this range) on reset.
global offset adjust
Global offset adjust is provided by an adder using three independent bipolar offset coefficients set through the
serial interface. A range of ±4 times the ADC full-scale range in steps of a half output LSB is provided. This range
allows the output signal to be shifted across the entire range of the 16-bit output bus. The global offset
coefficients are programmed as 16-bit 2s complement numbers, which default to zero on reset.
global gain adjust
Global gain adjust is provided by a multiplier using gain values set through the serial interface. Three
independent 16-bit gain values with a range of 0 to 2 are stored (one for each channel). The default value of
the global gain coefficients is 1 (equivalent to decimal 32768 in this range).
threshold detector
The threshold detector operates on the output signal from the global gain adjust stage, comparing the signal
to individual threshold levels for each color channel, which are programmed through the serial interface. If the
signal exceeds the threshold, the DETOP terminal is forced high. Two basic modes of operation can be
programmed, either multiplexing between the three channels in sequence with the internal data, or operating
continuously on one of the three channels. The input signals to the threshold detector are represented as 16-bit
bipolar 2s complement numbers. Threshold values should be programmed as 15-bit unipolar numbers in the
range 0 to 32767.
effect of image processing on ADC output
The combined effect of the image processing sections on the ADC output is summarized by the formula in the
note following Table 4. All values are shown in decimal. Examples of the process are given in Table 4. Examples
1–8 show the results with no pixel offset scaling. Examples 9–11 show the added effect of pixel offset scaling.
All examples use a half range ADC value (2048) for the ADC output (ADCOP).
If defaults are used throughout, then the output of the ADC is output directly on the OP0–OP11 bus as listed
in Table 4, example 1.
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265
21