WM8224 datasheet(26/50 Pages) CIRRUS

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WM8224

Production Data

PD, Rev 4.2, September 2013

PROGRAMMABLE AUTOMATIC BLACK LEVEL CALIBRATION

The Programmable Automatic Black-Level Calibration (BLC) function is to adjust the D.C. offset of the

output data such that the digital output code for black pixels is calibrated to a target black level value.

The D.C. offset is determined during the optically-black pixels at the beginning of the linear sensor,

and removed during the image-pixels as shown in Figure 22.

Black Pixel Period

Image Pixel Period

Determine Black

Level Offset

Remove Black Level Offset

from Image Pixels

Figure 22 Linear Sensor Model

The automatic black level calibration operates assuming 12 bits ADC resolution. Adjustments to

calculations must be made for different ADC resolutions.

The black level calibration process occurs in two stages as shown in Figure 23 below:



Coarse Adjust Calibration: This is a mixed signal loop which removes the coarse offset by

adjusting the offset DAC.



Fine Adjust Calibration: This is a digital loop which removes the remaining offset with

better noise tolerance, utilising ADC over-range to improve the dynamic range of the

system.

TARGET BL

Input Black Level V1

Adjusted

ADC Output

PGA

ADC

Offset

DAC

Mixed

Signal

LOOP

Digits

Coarse Adjust

Calibration

Fine Adjust

Calibration

Digital

LOOP

Digits

Figure 23 BLC Top-Level Circuitry

TARGET CODES

The user must specify a target black level for each Red, Green and Blue channel through the

registers TARGETR, TARGETG and TARGETB. If, during the black-pixel period, the average ADC

output code was, for example, 100 and the user specified the target black level code to be 10, the

BLC circuitry would determine 90 codes should be subtracted from the ADC output. These 90 codes

will then be subtracted from every image-pixel code output from the ADC.

Note – changing the PGA gain affects the black-level through the device; the gain should therefore

not be changed during a BLC procedure. If the PGA gain changes, then the BLC routine should be

re-run.

The automatic black level calibration feature operates with the assumption of a 12bit ADC resolution.

The register settings for Target Codes (TARGETx[7:0]) should be set differently depending on the

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