AD8322: 5 V CATV Line Driver with Coarse Step Output Power Control 0 pages

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a
AD8322
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., 2000
5 V CATV Line Driver
Coarse Step Output Power Control
FUNCTIONAL BLOCK DIAGRAM
DECODE
SHIFT
REGISTER
DATA LATCH
AD8322
VIN+
VIN–
VOUT+
VOUT–
BUFFER
R1
R2
ATTENUATION
CORE
8
8
8
POWER-DOWN
LOGIC
POWER
AMP
DATEN DATA CLK GND (12 PINS) PD
ZIN (SINGLE) = 210
ZIN (DIFF) = 235
ZOUT DIFF =
75
VCC (7 PINS)
DIFF
OR SINGLE
INPUT
AMP
FEATURES
Supports DOCSIS Standard for Reverse Path
Transmission
Gain Programmable in 6 dB Steps Over a 42 dB
Range
Low Distortion at 60 dBmV Output
–58 dBc SFDR at 21 MHz
–56 dBc SFDR at 42 MHz
Output Noise Level
–46 dBmV in 160 kHz Bandwidth
Maintains 75 Output Impedance
Power-Up and Power-Down Condition
180 MHz Bandwidth
5 V Supply Operation
Supports SPI Interfaces
APPLICATIONS
Gain Programmable Line Driver
DOCSIS-Compliant Data Modems
Interactive Set-Top Boxes
PC Plug-in Modems
General-Purpose Digitally Controlled Variable Gain Block
GENERAL DESCRIPTION
The AD8322 is a low-cost, digitally controlled variable gain amplifier
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 42.14 dB range, with gain steps of 6.02 dB/major
carry.
The AD8322 comprises a digitally controlled variable attenuator
of 0 dB to –42.14 dB, which is preceded by a low-noise, fixed-gain
buffer and is followed by a low-distortion, high-power amplifier.
The AD8322 accepts a differential or single-ended input
signal. The output is specified for driving a 75 Ù load, such as
coaxial cable.
Distortion performance of –58 dBc is achieved with an output level
up to 60 dBmV at 21 MHz bandwidth. A key performance and
cost advantage of the AD8322 results from the ability to maintain
a constant 75 Ù output impedance during power-up and powerdown
conditions. This eliminates the need for external 75 Ù
termination resulting in twice the effective output voltage when
compared to a standard operational amplifier.
The AD8322 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.
GAIN CODE – Decimal
1
–55
DISTORTION – dBc
–60
–65
–70
–75
2 4 8 16 32 64 128
fO = 42MHz
VO = 60dBmV
@ MAX GAIN
HD3
HD2
Figure 1. Harmonic Distortion vs. Gain Control