AD9821 datasheet(11/16 Pages) AD

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REV. 0

AD9821

–11–

CIRCUIT DESCRIPTION AND OPERATION

The AD9821 signal processing chain is shown in Figure 10.

Each processing step is essential in achieving a high quality

image from the raw imager pixel data.

Differential Input SHA

The differential input SHA circuit is designed to accommodate

a variety of different image sensor output voltages. The timing

shown in Figure 8 illustrates how the DATACLK signal is used to

sample both the VIN+ and VIN– signals simultaneously. The

imager signal is sampled on the rising edges of DATACLK.

Placement of this clock signal is critical in achieving the best

performance from the imager. An internal DATACLK delay (tID)

of 3 ns is caused by internal propagation delays.

The differential input can be used in a variety of single-ended

and differential configurations, as shown in Table VI. The

allowable voltage range for both VIN+ or VIN– is from 0 V

to 1.8 V. Signal levels outside this range will result in severely

degraded performance. Regardless of the input configuration,

the voltage sampled by the SHA is always equal to VIN+ minus

VIN–. VIN+ must always be equal to or greater than VIN– or

0dB TO 36dB

BYP1

VIN+

DIGITAL

FILTERING

CLPOB

OPTICAL BLACK

CLAMP

DOUT

12-BIT

ADC

VGA

8-BIT

DAC

CLAMP LEVEL

VGA GAIN

SHA

INTERNAL

VREF

REFT

REFB

1.0V

2.0V

DATACLK

VIN–

0.45V

INTERNAL

BIAS

PBLK

0.1 F

1.0 F 1.0 F

Figure 10. Internal Block Diagram

negative clipping will occur. A small amount of offset between

the VIN+ and VIN– signals is allowable and can be corrected by

the Optical Black Clamp, up to

±30 mV.

Note that the VIN+ and VIN– inputs do not contain any dc

restoration or bias circuitry. Therefore, dc-coupling is recom-

mended when driving the AD9821 analog inputs. If ac-coupling is

used, external biasing circuitry must be provided for the VIN+

and VIN– inputs to keep them in the acceptable common-mode

voltage range of 0 V to 1.8 V.

Table VI. Example Input Voltage Configurations

VIN+ Range (V) VIN– Range (V) SHA Output Range (V)

Black White

Black

White

Black

White

1.0

0.5

1.5

0.5

1.0

1.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

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