a
5 V CATV Line Driver Fine Step
Output Power Control
AD8325
FEATURES
FUNCTIONAL BLOCK DIAGRAM
Supports DOCSIS Standard for Reverse Path
VCC (7 PINS)
BYP
Transmission
Gain Programmable in 0.75 dB Steps Over a 59.45 dB
Range
Low Distortion at 61 dBmV Output
VIN+
–57 dBc SFDR at 21 MHz
DIFF OR
SINGLE
INPUT
AMP
ATTENUATION
CORE
ZOUT DIFF =
75
8
ZIN (SINGLE) = 800
ZIN (DIFF) = 1.6k
R1
R2
AD8325
VERNIER
DECODE
DATA LATCH
SHIFT
REGISTER
POWER-DOWN
LOGIC
POWER
AMP
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
–55 dBc SFDR at 42 MHz
VIN–
VOUT+
VOUT–
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
–50
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.
–52
The AD8325 comprises a digitally controlled variable attenuator
–54
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
W load, such as
coaxial cable.
DISTORTION
–
dBc
–56
–58
Distortion performance of –57 dBc is achieved with an output
–60
level up to 61 dBmV at 21 MHz bandwidth. A key performance
–62
and cost advantage of the AD8325 results from the ability to
maintain a constant 75
W output impedance during Transmit
–64
DATEN
DATACLK
GND (11 PINS)
TXEN
SLEEP
VOUT
VOUT = 62dBmV
@ MAX GAIN
= 61dBmV
@ MAX GAIN
VOUT = 60dBmV
@ MAX GAIN
VOUT = 59dBmV
@ MAX GAIN
Enable and Transmit Disable conditions. In addition, this
5
15
25
35
45
55
device has a sleep mode function that reduces the quiescent
FUNDAMENTAL FREQUENCY – MHz
current to 4 mA.
Figure 1. Worst Harmonic Distortion vs. Gain Control
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.
REV. A
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties that
may result from its use. No license is granted by implication or otherwise
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
under any patent or patent rights of Analog Devices. Trademarks and
Tel: 781/329-4700
registered trademarks are the property of their respective owners.
Fax: 781/461-3113
2005 Analog Devices, Inc. All rights reserved.
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