REV. 0

AD9845A

–18–

RR

Gb

Gb

Gr

Gr

BB

CCD: PROGRESSIVE BAYER

LINE0

GAIN0, GAIN1, GAIN0, GAIN1 ...

RR

Gr

Gr

Gb

Gb

BB

LINE1

LINE2

GAIN2, GAIN3, GAIN2, GAIN3 ...

GAIN0, GAIN1, GAIN0, GAIN1 ...

MOSAIC SEPARATE COLOR

STEERING MODE

Figure 26. CCD Color Filter Example: Progressive Scan

LINE0

GAIN0, GAIN1, GAIN0, GAIN1 ...

RR

Gr

Gr

LINE1

LINE2

GAIN0, GAIN1, GAIN0, GAIN1 ...

GAIN0, GAIN1, GAIN0, GAIN1 ...

Gb

Gb

BB

LINE0

GAIN2, GAIN3, GAIN2, GAIN3 ...

LINE1

LINE2

GAIN2, GAIN3, GAIN2, GAIN3 ...

GAIN2, GAIN3, GAIN2, GAIN3 ...

CCD: INTERLACED BAYER

EVEN FIELD

VD SELECTED COLOR

STEERING MODE

ODD FIELD

Gb

Gb

BB

Gb

Gb

BB

Gb

Gb

BB

RR

Gr

Gr

RR

Gr

Gr

RR

Gr

Gr

Figure 27. CCD Color Filter Example: Interlaced

The same Bayer pattern can also be interlaced, and the VD

Selected mode should be used with this type of CCD (see Fig-

ure 27). The Color Steering performs the proper multiplexing of

the R, G, and B gain values (loaded into the PxGA gain regis-

ters), and is synchronized by the user with vertical (VD) and

horizontal (HD) sync pulses. For more detailed information, see

the PxGA Timing section. The PxGA gain for each of the four

channels is variable from –2 dB to +10 dB, controlled in 64 steps

through the serial interface. The PxGA gain curve is shown in

Figure 28.

PxGA GAIN REGISTER CODE

10

32

40

48

56

0

8

16

24

31

8

6

4

2

0

–2

(100000)

(011111)

Figure 28. PxGA Gain Curve

Variable Gain Amplifier

The VGA stage provides a gain range of 2 dB to 36 dB, program-

mable with 10-bit resolution through the serial digital interface.

Combined with 4 dB from the PxGA stage, the total gain range

for the AD9845A is 6 dB to 40 dB. The minimum gain of 6 dB

is needed to match a 1 V input signal with the ADC full-scale

range of 2 V. When compared to 1 V full-scale systems (such as

ADI’s AD9803), the equivalent gain range is 0 dB to 34 dB.

The VGA gain curve is divided into two separate regions. When

the VGA Gain Register code is between 0 and 511, the curve

follows a (1 + x)/(1 – x) shape, which is similar to a “linear-in-

dB” characteristic. From code 512 to code 1023, the curve follows

a “linear-in-dB” shape. The exact VGA gain can be calculated

for any Gain Register value by using the following two equations:

Code Range

Gain Equation (dB)

0–511

512 –1023

Gain = (0.0354)(code) – 0.4

As shown in the CCD Mode Specifications, only the VGA gain

range from 2 dB to 36 dB has tested and guaranteed accuracy.

This corresponds to a VGA gain code range of 91 to 1023. The

Gain Accuracy Specifications also include the PxGA gain of 4 dB,

for a total gain range of 6 dB to 40 dB.

VGA GAIN REGISTER CODE

36

0

127

255

383

511

639

767

895

1023

30

24

18

12

6

0

Figure 29. VGA Gain Curve (Gain from PxGA Not Included)

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. Any value between 0 LSB and 255 LSB may be pro-

grammed, with 8-bit resolution. 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 clamping is used during the post processing,

the AD9845A optical black clamping may be disabled using Bit

D5 in the Operation Register (see Serial Interface Timing and

Internal Register Description section). When the loop is disabled,

the Clamp Level Register may still be used to provide program-

mable offset adjustment.

Horizontal timing is shown in Figure 6. 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 pulsewidths may be

used, but clamp noise may increase, and the ability to track

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