AD9849 datasheet(30/36 Pages) AD

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AD9848/AD9849

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Optical Black Clamp

The optical black clamp loop is used to remove residual offsets in

the signal chain and to track low-frequency variations in the CCD’s

black level. During the optical black (shielded) pixel interval on

each line, the ADC output is compared with a fixed black level

reference, selected by the user in the Clamp Level Register. The

value can be programmed between 0 LSB and 63.75 LSB on the

AD9848, and between 0 LSB and 255 LSB on the AD9849. The

clamp level can be programmed with 8-bit resolution. The result-

ing error signal is filtered to reduce noise, and the correction value

is applied to the ADC input through a D/A converter. Normally,

the optical black clamp loop is turned on once per horizontal line,

but this loop can be updated more slowly to suit a particular

application. If external digital clamping is used during the post

processing, the AD9848/AD9849 optical black clamping may be

disabled using Bit D2 in the OPRMODE Register. When the loop

is disabled, the Clamp Level Register may still be used to provide

programmable offset adjustment.

The CLPOB pulse should be placed during the CCD’s optical

black pixels. It is recommended that the CLPOB pulse duration

be at least 20 pixels wide to minimize clamp noise. Shorter pulse-

widths may be used, but clamp noise may increase, and the ability

to track low-frequency variations in the black level will be reduced.

See the section on Horizontal Clamping and Blanking and also

the Applications Information section for timing examples.

A/D Converter

The AD9848/AD9849 uses high-performance 10-bit/12-bit

ADC architecture, optimized for high speed and low power.

Differential Nonlinearity (DNL) performance is typically better

than 0.5 LSB. The ADC uses a 2 V input range. Better noise

performance results from using a larger ADC full-scale range.

See TPC 1–TPC 4 for typical linearity and noise performance

plots for the AD9848/AD9849.

AD9849 датащи(PDF) 30 Page - Analog Devices