AD9803 datasheet(11/19 Pages) AD

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AD9803

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THEORY OF OPERATION

Introduction

The AD9803 is a 10-bit analog-to-digital interface for CCD

cameras. The block level diagram of the system is shown in

Figure 23. The device includes a correlated double sampler

(CDS), 0 dB–30 dB programmable gain amplifier (PGA), black

level correction loop, input clamp and voltage reference. The

only external analog circuitry required at the system level is an

emitter follower buffer between the CCD output and AD9803

inputs.

CLPDM

INPUT CLAMP

DIN

CDS

SHA

ADC

INTEG

BLACK LEVEL CLAMP

PGA

DIFFERENTIAL SIGNAL PATH

CLPOB

Figure 23. CCD Mode Signal Path

Correlated Double Sampling (CDS)

CDS is important in high performance CCD systems as a

method for removing several types of noise. Basically, two

samples of the CCD output are taken: one with the signal

present (“data”) and one without (“reference”). Subtracting

these two samples removes any noise which is common—or

correlated—to both.

Figure 24 shows the block diagram of the AD9803’s CDS. The

S/H blocks are directly driven by the input and the sampling

function is performed passively, without the use of amplifiers.

This implementation relies on the off-chip emitter follower

buffer to drive the two 10 pF sampling capacitors. Only one

capacitor at a time is seen at the input pin.

10pF

S/H

OUT

FROM

CCD

Figure 24. CDS Block Diagram

The AD9803 actually uses two CDS circuits in a “ping pong”

fashion to allow the system more acquisition time. In this way,

the output from one of the two CDS blocks will be valid for an

entire clock cycle. Thus, the bandwidth requirement of the

subsequent gain stage is reduced as compared to that for a single-

channel CDS system. This lower bandwidth translates to lower

power and noise.

Programmable Gain Amplifier (PGA)

The on-chip PGA provides a gain range of 0 dB–30 dB, which

is “linear in dB.” Typical gain characteristics are shown in

Figures 25 and 26.

PGACONT1 – Volts

–5

0.5

1.0

1.5

2.0

2.5

3.0

Figure 25. PGA Gain Curve—Analog Control

PGA GAIN REGISTER

–5

1023

171

341

511

682

852

Figure 26. PGA Gain Curve—Digital Control

As shown in Figure 27, analog PGA control is provided through

the PGACONT1 and PGACONT2 inputs. PGACONT1 pro-

vides coarse and PGACONT2 fine (1/16) gain control. The

PGA gain can also be controlled using the internal 10-bit DAC

through the serial digital interface. The gain characteristic

shown in Figure 26, with the internal DAC providing the same

control range as PGACONT1. See the Serial Interface Specifi-

cations for more details.

PGACONT1

PGACONT2

PGACONT1 = COARSE CONTROL

PGACONT2 = FINE (1/16) CONTROL

Figure 27. Analog PGA Control

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