AD9847 datasheet(24/28 Pages) AD

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REV. A

AD9847

–24–

COLOR

STEERING

CONTROL

4:1

MUX

GAIN0

GAIN1

GAIN2

GAIN3

PxGA

PxGA STEERING

MODE

SELECTION

PxGA GAIN

REGISTERS

CONTROL

BITS D0–D2

SHP/SHD

VGA

CDS

Figure 17. PxGA Block Diagram

RGr

GAIN0, GAIN1, GAIN0, GAIN1...

LINE0

GAIN2, GAIN3, GAIN2, GAIN3...

LINE1

GAIN0, GAIN1, GAIN0, GAIN1...

LINE2

CCD: PROGRESSIVE BAYER

MOSAIC SEPARATE COLOR

STEERING MODE

Figure 18a. CCD Color Filter Example: Progressive Scan

RGr

GAIN0, GAIN1, GAIN0, GAIN1...

LINE0

GAIN0, GAIN1, GAIN0, GAIN1...

LINE1

GAIN0, GAIN1, GAIN0, GAIN1...

LINE2

CCD: INTERLACED BAYER

EVEN FIELD

VD SELECTED COLOR

STEERING MODE

GAIN2, GAIN3, GAIN2, GAIN3...

LINE0

GAIN2, GAIN3, GAIN2, GAIN3...

LINE1

GAIN2, GAIN3, GAIN2, GAIN3...

LINE2

ODD FIELD

Figure 18b. 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 Figure 18b).

The color steering performs the proper multiplexing of the R, G,

and B gain values (loaded into the PxGA gain registers) 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 varies from

–2 dB to +10 dB, controlled in 64 steps through the serial inter-

face. The PxGA gain curve is shown in Figure 19.

PxGA GAIN REGISTER CODE

–2

(011111)

(100000)

Figure 19. 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 AD9847 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 character-

istic. 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

Gain = 20 log10 ([658

code] / [658 – code]) – 0.4

512–1023

Gain = (0.0354)(code) – 0.04

VGA GAIN REGISTER CODE

127

255

383

511

639

767

895

1023

Figure 20. VGA Gain Curve (Gain from PxGA Not Included)

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