CS7622 datasheet(10/36 Pages) CIRRUS

10 / 36 page

CS7622

DS322PP1

matically biased to mid-supply without worrying

about external circuitry to perform this task.

3.2 Black Level Adjustment

In order to maintain a constant reference level for

black pixels, a feedback loop is implemented that

sets the black level value at the output of the ADC

to 64 in the 13 bit digital code. This loop is active

during the optically black pixels which are output

at the beginning and end of a frame as well as dur-

ing a portion of the horizontal blanking period. The

presence of black pixels in the CCD output is indi-

cated by the CLAMP pulse, which is supplied ex-

ternally through the CLAMP pin. The black level

can also be written to through the serial port.

In order to acquire a starting value for the black lev-

el, the loop will run over the several lines of black

pixels at the beginning of the frame. The block di-

agram of the loop is shown in Figure 10. The up-

date rate is once per line during active pixel lines as

long as the Clamp pulse is < n+10 cycles. Where n

is the number of pixels accumulated before the

black loop is updated and is programmable through

than n+10 cycles the black loop is updated every

n+10 cycles. For example, during optical back lines

the loop is updated several times at a rate of once

every n+10 cycles.

The open-loop transfer function of the black level

adjustment loop is

blk_gain = 1, 2, 4, or 8

where blk_gain is programmable through a register

and n = # of black pixels during clamp time, which

is also programmable. The value of Kxn will deter-

mine the open-loop gain of the system. The settling

time for the loop can be calculated using the fol-

lowing formula:

For offset range=1 (reg 06h, bit 0)

For offset range =0

CCD

INPUT

SIGNAL

ck_ft

ck_data

OUT OF

STAGE 1

OUT OF

STAGE 2

OUT OF

STAGE 3

Figure 9. Idealized timing diagram of VGA/CDS circuit

V(1)

V(2)

V(3)

V(1)

V(2)

V(3)

V(1)

V(2)

V(3)

V(1)

V(2)

()

–

-------------

256

--------- blk_gain

1nK

–

()

---------------------------

–



 1

-----

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

-------

–





----------------------------

–







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



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