AD8321 Datasheet, PDF (1/19 Pages) Analog Devices – Gain Programmable CATV Line Driver

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FEATURES
Linear in dB Gain Response Over >53 dB Range
Drives Low Distortion >11 dBm Signal into 75 ⍀ Load:
–53 dBc SFDR at 42 MHz
Very Low Output Noise Level
Maintains Constant 75 ⍀ Output Impedance
Power-Up and Power-Down Condition
No Line Transformer Required
Upper Bandwidth: 235 MHz (Min Gain)
9 V Single Supply Operation
Power-Down Functionality
Supports SPI Interface
Low Cost
APPLICATIONS
Gain Programmable Line Driver
HFC High Speed Data Modems
Interactive CATV Set-Top Boxes
CATV Plant Test Equipment
General Purpose IF Variable Gain Block
Gain Programmable
CATV Line Driver
AD8321
FUNCTIONAL BLOCK DIAGRAM
VCC
GND
VIN+
VIN–
AD8321
INV
ATTENUATOR CORE
PWR
AMP
REVERSE
AMP
VOUT
DATADASTHAIFLTARTECGHISTER
POWER-
DOWN/
PD
SWITCH
DATA SHIFT REGISTER
INTER
DATEN CLK
SDATA
DESCRIPTION
The AD8321 is a low cost digitally controlled variable gain
amplifier optimized for coaxial line driving applications such as
cable modems that are designed to the DOCSIS* (upstream)
standard. An 8-bit serial word determines the desired output
gain over a 53.4 dB range, resulting in gain changes of 0.75 dB/
LSB.
The AD8321 comprises a digitally controlled variable attenuator
of 0 dB to –53.4 dB, which is preceded by a low noise, fixed
gain buffer and followed by a low distortion high power ampli-
fier. The AD8321 accepts a differential or single-ended input
signal. The output is specified for driving a 75 Ω load, such as
coaxial cable, although the AD8321 is capable of driving other
loads. Performance of –53 dBc is achieved with an output level
up to 11 dBm at 42 MHz bandwidth using a 9 V supply.
A key performance and cost advantage of the AD8321 results
from the ability to maintain a constant 75 Ω output impedance
during power-up and power-down conditions. This eliminates
the need for external 75 Ω termination, resulting in twice the
effective output voltage when compared to a standard opera-
tional amplifier, thus eliminating the need for a transformer.
*Data-Over-Cable Service Interface Specifications
The AD8321 is packaged in a low cost 20-lead SOIC, operates
from a single +9 V supply, and has an operational temperature
range of –40°C to +85°C.
–40
f_O = 42MHz
V_IN = 137mV p-p
(P_IN = –15dBm)
–50
(P_OUT = 11dBm @
MAX GAIN)
–60
HD3
–70
HD2
–80
–90
0
8 16 24 32 40 48 56 64 72
GAIN CONTROL – Decimal
Figure 1. Harmonic Distortion vs. Gain Control
REV. 0
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© Analog Devices, Inc., 1999