CS7620 datasheet(33/70 Pages) CIRRUS

33 / 70 page

CS7620

DS301PP2

4.8

General Black Level

The black loop is a feedback system that causes the ADC output to settle to 64 during the register defined black

pixels. This has the purpose of removing any CCD and system offsets and defining 64 as the known black level.

The loop has an exponential settling response and the time constant of this loop is effected by the black loop gain

and the number of black pixels to accumulate before updating the black accumulator. See Figure 10 for a block

diagram of the black level.

4.9

Black Level Control - Loop Gain, Clamp Length

Default = 0Ah; Read/Write (address 0Fh).

Where:

K = 1/256*blk_gain

n = Black loop clamp length = blk_clp_l[5:0]

fu = update rate

Bit Number

765432

Bit Name

blk_gain1

blk_gain0

blk_clp_15

blk_clp_14

blk_clp_13

blk_clp_12

blk_clp_11

blk_clp_10

Default

0001010

Bit

Mnemonic

Function

blk_gain1

Black Loop Gain Factor: The Black loop gain factor can be set to 1x,2x,4x,or

8x and is simply a multiplying constant to effect the weight of each black pixel

before it is accumulated.

00 - defines a gain of 1x

01 - defines a gain of 2x

10 - defines a gain of 4x

11 - defines a gain of 8x

blk_gain0

blk_clp_15

Black Loop Clamp Length: The black clamp length effects the loop time con-

stant and also acts to average out noise in the black level. The larger this val-

ue the more pixels that are summed before the loop is updated which causes

greater averaging and a smaller settling time constant.

Table 11 shows the black loop time constant for various settings of Offset

Range (register 06h, bit 0) and Fixed Gain Settings (register 16h, bits 5-3).

blk_clp_14

blk_clp_13

blk_clp_12

blk_clp_11

blk_clp_10

Fixed Gain

(Register 16h)

Offset Range = 1

Offset Range = 0

not fixed

-1/(ln(1-nK))(1/fu)

-1/(ln(1-nK/2))(1/fu)

-1/(ln(1-nK/8))(1/fu)

-1/(ln(1-nK/16))(1/fu)

-1/(ln(1-nK/4))(1/fu)

-1/(ln(1-nK/8))(1/fu)

-1/(ln(1-nK/2))(1/fu)

-1/(ln(1-nK/4))(1/fu)

-1/(ln(1-nK))(1/fu)

-1/(ln(1-nK/2))(1/fu)

Table 11. Black Loop Time Constant

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