5 V CATV Line Driver Fine Step
Output Power Control
aꢀ
AD8325
FEATURES
FUNCTIONAL BLOCK DIAGRAM
Supports DOCSIS Standard for Reverse Path
Transmission
V
CC
(7 PINS)
BYP
Gain Programmable in 0.75 dB Steps Over a 59.45 dB
Range
R1
AD8325
Low Distortion at 61 dBmV Output
–57 dBc SFDR at 21 MHz
–55 dBc SFDR at 42 MHz
V
V
V
V
IN+
OUT+
DIFF OR
SINGLE
INPUT
AMP
POWER
AMP
ATTENUATION
CORE
VERNIER
IN–
OUT–
Output Noise Level
–48 dBmV in 160 kHz
Z
DIFF =
OUT
75�
8
DECODE
R2
Maintains 75 � Output Impedance
Transmit Enable and Transmit Disable Modes
Upper Bandwidth: 100 MHz (Full Gain Range)
5 V Supply Operation
Z
Z
(SINGLE) = 800�
(DIFF) = 1.6k�
IN
IN
8
POWER-DOWN
LOGIC
DATA LATCH
8
SHIFT
REGISTER
Supports SPI Interfaces
APPLICATIONS
Gain-Programmable Line Driver
DOCSIS High-Speed Data Modems
Interactive Cable Set-Top Boxes
PC Plug-in Cable Modems
TXEN
DATEN DATA CLK GND (11 PINS)
SLEEP
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.
–50
–52
–54
–56
–58
–60
–62
–64
V
= 62dBmV
OUT
@ MAX GAIN
V
= 61dBmV
OUT
@ MAX GAIN
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 W 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 W 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.
V
= 60dBmV
OUT
@ MAX GAIN
V
= 59dBmV
OUT
@ MAX GAIN
5
15
25
35
45
55
65
FUNDAMENTAL FREQUENCY – MHz
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
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