AD8325 Datasheet, PDF (1/16 Pages) Analog Devices – 5 V CATV Line Driver Fine Step Output Power Control

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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
5 V CATV Line Driver Fine Step
Output Power Control
AD8325
FUNCTIONAL BLOCK DIAGRAM
V_CC (7 PINS)
BYP
R1
AD8325
V_IN+
V_IN–
DIFF OR
SINGLE
INPUT
AMP
VERNIER
R2
Z_IN (SINGLE) = 800⍀
Z_IN (DIFF) = 1.6k⍀
ATTENUATION
CORE
8
DECODE
POWER
AMP
Z_OUT DIFF =
75⍀
V_OUT+
V_OUT–
8
DATA LATCH
POWER-DOWN
LOGIC
8
SHIFT
REGISTER
DATEN DATA CLK GND (11 PINS) TXEN SLEEP
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.
–50
V_OUT = 62dBmV
@ MAX GAIN
–52
V_OUT = 61dBmV
@ MAX GAIN
–54
–56
–58
–60
–62
–64
5
V_OUT = 60dBmV
@ MAX GAIN
V_OUT = 59dBmV
@ MAX GAIN
15
25
35
45
55
65
FUNDAMENTAL FREQUENCY – MHz
Figure 1. Worst Harmonic Distortion vs. Gain Control
REV. 0
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© Analog Devices, Inc., 2001