AD9822

Rev. B | Page 16 of 20

ANALOG INPUTS—SHA MODE

Figure 12 shows the analog input configuration for the SHA

mode of operation. Figure 13 shows the internal timing for the

sampling switches. The input signal is sampled when CDSCLK2

transitions from high to low, opening S1. The voltage on the

OFFSET pin is also sampled on the falling edge of CDSCLK2,

when S2 opens. S3 is then closed, generating a differential

output voltage representing the difference between the sampled

input voltage and the OFFSET voltage. The input clamp is

disabled during SHA mode operation.

AD9822

S1

2pF

S3

CML

VINR

INPUT SIGNAL

S2

2pF

CML

OFFSET

RED

VING

GREEN

VINB

BLUE

OPTIONAL DC OFFSET

(OR CONNECT TO GND)

Figure 12. SHA Mode Input Configuration (All Three Channels are Identical)

CDSCLK2

Q3

(INTERNAL)

S1, S2 CLOSED

S1, S2 CLOSED

S3 CLOSED

S3 CLOSED

S3 OPEN

S1, S2 OPEN

Figure 13. SHA Mode Internal Switch Timing

Figure 14 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 is restored to 0. 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

µF

AVDD

VREF FROM

CIS MODULE

DC OFFSET

R1

R2

SHA

SHA

SHA

Figure 14. SHA Mode Used with External DC Offset

PROGRAMMABLE GAIN AMPLIFIERS (PGA)

The AD9822 uses one PGA for each channel. Each PGA has a

gain range from 1× (0 dB) to 5.8× (15.5 dB), adjustable in

64 steps. Figure 15 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

⎥⎦

⎤

⎢⎣

⎡

−

+

=

63

63

4.7

1

5.7

G

Gain

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

varies from 0 to 63.

1.0

5.7

4.0

5.0

2.0

3.0

PGA REGISTER VALUE (Decimal)

63

0

4

8

121620 2428 323640 444852 5660

15

12

9

6

3

0

Figure 15. PGA Gain Transfer Function