REV. 0

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use, nor for any infringements of patents or other rights of third parties

which may result from its use. No license is granted by implication or

otherwise under any patent or patent rights of Analog Devices.

a

AD8321

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 1999

Gain Programmable

CATV Line Driver

FEATURES

Linear in dB Gain Response Over >53 dB Range

Drives Low Distortion >11 dBm Signal into 75

Load:

–53 dBc SFDR at 42 MHz

Very Low Output Noise Level

Maintains Constant 75

Output Impedance

Power-Up and Power-Down Condition

No Line Transformer Required

Upper Bandwidth: 235 MHz (Min Gain)

9 V Single Supply Operation

Power-Down Functionality

Supports SPI Interface

Low Cost

APPLICATIONS

Gain Programmable Line Driver

HFC High Speed Data Modems

Interactive CATV Set-Top Boxes

CATV Plant Test Equipment

General Purpose IF Variable Gain Block

DESCRIPTION

The AD8321 is a low cost digitally controlled variable gain

amplifier optimized for coaxial line driving applications such as

cable modems that are designed to the DOCSIS* (upstream)

standard. An 8-bit serial word determines the desired output

gain over a 53.4 dB range, resulting in gain changes of 0.75 dB/

LSB.

The AD8321 comprises a digitally controlled variable attenuator

of 0 dB to –53.4 dB, which is preceded by a low noise, fixed

gain buffer and followed by a low distortion high power ampli-

fier. The AD8321 accepts a differential or single-ended input

signal. The output is specified for driving a 75

Ω load, such as

coaxial cable, although the AD8321 is capable of driving other

loads. Performance of –53 dBc is achieved with an output level

up to 11 dBm at 42 MHz bandwidth using a 9 V supply.

A key performance and cost advantage of the AD8321 results

from the ability to maintain a constant 75

Ω output impedance

during power-up and power-down conditions. This eliminates

the need for external 75

Ω termination, resulting in twice the

effective output voltage when compared to a standard opera-

tional amplifier, thus eliminating the need for a transformer.

*Data-Over-Cable Service Interface Specifications

FUNCTIONAL BLOCK DIAGRAM

AD8321

POWER-

DOWN/

SWITCH

INTER

ATTENUATOR CORE

INV

DATA SHIFT REGISTER

DATA SHIFT REGISTER

DATA LATCH

PWR

AMP

DATEN CLK

VIN+

VIN–

PD

VOUT

REVERSE

AMP

GND

VCC

SDATA

The AD8321 is packaged in a low cost 20-lead SOIC, operates

from a single +9 V supply, and has an operational temperature

range of –40

°C to +85°C.

GAIN CONTROL – Decimal

–90

0

8

16

24

32

40

48

56

64

72

–80

–70

–60

–50

–40

MAX GAIN)

HD3

HD2

Figure 1. Harmonic Distortion vs. Gain Control