AD9991 datasheet(32/60 Pages) AD

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AD9991

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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 inter-

val 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 in

256 steps. The resulting 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 clamp-

ing is used during the postprocessing, the AD9991 optical black

clamping may be disabled using Bit D2 in the OPRMODE regis-

ter. 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 Horizontal Clamping and Blanking section and the Hori-

zontal Timing Sequence Example section for timing examples.

Digital Data Outputs

The AD9991 digital output data is latched using the DOUT

PHASE register value, as shown in Figure 33. Output data timing

is shown in Figure 8. It is also possible to leave the output latches

transparent so that the data outputs are valid immediately from

the A/D converter. Programming the AFE CONTROL register bit

D4 to a 1 will set the output latches transparent. The data outputs

can also be disabled (three-stated) by setting the AFE CONTROL

The data output coding is normally straight binary, but the coding

my be changed to gray coding by setting the AFE CONTROL

REV. 0

AD9991 Datasheet(PDF) 32 Page - Analog Devices