REV. A

–13–

AD9822

Figure 12 shows how the OFFSET pin may be used in a CIS

application for coarse offset adjustment. Many CIS signals have

dc offsets ranging from several hundred millivolts to more than

1 V. By connecting the appropriate dc voltage to the OFFSET

pin, the CIS signal will be restored to “zero.” After the large dc

offset is removed, the signal can be scaled using the PGA to

maximize the ADC’s dynamic range.

AD9822

OFFSET

RED-OFFSET

GREEN-OFFSET

BLUE-OFFSET

VINR

VING

VINB

RED

GREEN

BLUE

0.1 F

AVDD

VREF FROM

CIS MODULE

DC OFFSET

R1

R2

SHA

SHA

SHA

Figure 12. SHA-Mode Used with External DC Offset

Programmable Gain Amplifiers

The AD9822 uses one Programmable Gain Amplifier (PGA) for

each channel. Each PGA has a gain range from 1x (0 dB) to

5.8x (15.5 dB), adjustable in 64 steps. Figure 6 shows the PGA

gain as a function of the PGA register code. Although the gain

curve is approximately “linear in dB”, the gain in V/V varies

nonlinearly with register code, following the equation:

Gain

G

=

+

−













57

14 7

63

63

.

.

where G is the decimal value of the gain register contents, and

varies from 0 to 63.

PGA REGISTER VALUE – Decimal

15

0

12

9

6

3

0

4

8

12 16 20 24 28 32 36 40 44 48 52 56 60 63

5.7

5.0

4.0

3.0

2.0

1.0

Figure 13. PGA Gain Transfer Function