LM98714 datasheet(39/115 Pages) TI1

39 / 115 page

LM98714

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SNAS254A – OCTOBER 2006 – REVISED JANUARY 2014

PIXEL AVERAGING

In order to obtain a snapshot of the current value for black (for comparison with the desired level of black) the

ADC output is sampled upon activation of BLKCLP. Since a single optical black pixel is unlikely to be an accurate

representation of the black level, a number of adjacent pixels are averaged. The number of pixels sampled is

programmable by the Pixel Averaging Bit[5:4] within the Black Level Clamp Control register. The ability to select

the number of pixels to be averaged (4, 8, 16, or 32 per line) provides greater flexibility allowing the LM98714 to

be used with different CCDs having differing number of black pixels.

TARGET BLACK LEVEL

The Target Black Level registers define a 10-bit word that specifies an ADC output (on the 12 bit level)

corresponding to the desired optical black output code (ignoring the four LSBs of the 16 Bit ADC output). In other

words, one Target Black Level LSB corresponds to sixteen ADC LSBs. Assertion of the BLKCLP signal activates

the digital black clamp loop and the black level is steered toward the value stored in the output black level

Once the correct number of pixels have been averaged, the value is subtracted from the Target Black Level and

an error value is produced.

OFFSET INTEGRATION

Each time the BLKCLP signal is activated, the average ADC output of several black pixels is compared to the

Target Black Level producing an error value. This error value is not directly added (or subtracted) to the Black

Level Offset register, rather, the value applied is a programmable fraction of this error. This has the effect of

slowing down the offset convergence resulting in a calculation for offset that is less susceptible to noise. The

scaling factor is stored in the Offset integration Bits[3:2] of the Black Level Clamp Control register. The scaling

values are divided-by-8, 16, 32, or 64. Divide-by-8 provides the quickest convergence of the loop (for use when

the number of lines available for calibration is limited) and Divide-by-64 the longest (for use when using an large

number of lines to converge).

LINE AVERAGING

The Auto Black level Correction Loop can be run for 15 lines, 31 lines, 63 lines, or infinite (every line). The Line

Averaging Bits[7:6] found in the lack Level Clamp Control register set the number of lines that the loop will run

after the Start of Scan. The recommended use of the Auto Black Level Correction Loop is in a calibration period

prior to moving the sensor down the page or during the first several lines of the page. By experimenting with the

Line Averaging and Offset Integration bits with no sensor illumination (black pixels), the proper settings for the

Auto Black Level Correction Loop are determined when the ADC output converges to the Target Black Level

value. If the loop converges with the 15, 31, or 63 line setting, the loop can remain enabled. The loop does not

update the Black Level Offset DAC once the number of lines since “Start of Scan” has passed. If the loop

requires more than 63 lines to converge (i.e. requires Line Averaging = infinite), it is recommended to disable the

loop after convergence has been reached. In the “infinite” setting, the loop will continuously update the Black

Level Offset registers as long as the loop is enabled throughout the entire scan.

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LM98714 Datasheet(PDF) 39 Page - Texas Instruments