REV. 0

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

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a

AD8325

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

Tel: 781/329-4700

www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 2001

5 V CATV Line Driver Fine Step

Output Power Control

FUNCTIONAL BLOCK DIAGRAM

DIFF OR

SINGLE

INPUT

AMP

ATTENUATION

CORE

75

8

8

8

R1

R2

AD8325

DATEN

DATA

CLK

GND (11 PINS)

TXEN

SLEEP

VERNIER

DECODE

DATA LATCH

SHIFT

REGISTER

POWER-DOWN

LOGIC

POWER

AMP

BYP

FEATURES

Supports DOCSIS Standard for Reverse Path

Transmission

Gain Programmable in 0.75 dB Steps Over a 59.45 dB

Range

Low Distortion at 61 dBmV Output

–57 dBc SFDR at 21 MHz

–55 dBc SFDR at 42 MHz

Output Noise Level

–48 dBmV in 160 kHz

Maintains 75

Output Impedance

Transmit Enable and Transmit Disable Modes

Upper Bandwidth: 100 MHz (Full Gain Range)

5 V Supply Operation

Supports SPI Interfaces

APPLICATIONS

Gain-Programmable Line Driver

DOCSIS High-Speed Data Modems

Interactive Cable Set-Top Boxes

PC Plug-in Cable Modems

General-Purpose Digitally Controlled Variable Gain Block

GENERAL DESCRIPTION

The AD8325 is a low-cost, digitally controlled, variable gain ampli-

fier optimized for coaxial line driving applications such as cable

modems that are designed to the MCNS-DOCSIS upstream

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

over a 59.45 dB range resulting in gain changes of 0.7526 dB/LSB.

The AD8325 comprises a digitally controlled variable attenuator

of 0 dB to –59.45 dB, which is preceded by a low noise, fixed

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

fier. The AD8325 accepts a differential or single-ended input

signal. The output is specified for driving a 75

Ω load, such as

coaxial cable.

Distortion performance of –57 dBc is achieved with an output

level up to 61 dBmV at 21 MHz bandwidth. A key performance

and cost advantage of the AD8325 results from the ability to

maintain a constant 75

Ω output impedance during Transmit

Enable and Transmit Disable conditions. In addition, this

device has a sleep mode function that reduces the quiescent

current to 4 mA.

The AD8325 is packaged in a low-cost 28-lead TSSOP, operates

from a single 5 V supply, and has an operational temperature

range of –40

°C to +85°C.

FUNDAMENTAL FREQUENCY – MHz

–50

5

–52

–54

–56

–60

–64

15

25

35

45

55

65

–62

–58

@ MAX GAIN

@ MAX GAIN

@ MAX GAIN

@ MAX GAIN

Figure 1. Worst Harmonic Distortion vs. Gain Control