Gain Programmable
a
CATV Line Driver
AD8321
FEATURES
FUNCTIONAL BLOCK DIAGRAM
Linear in dB Gain Response Over >53 dB Range
Drives Low Distortion >11 dBm Signal into 75
Load:
VCC
GND
–53 dBc SFDR at 42 MHz
APPLICATIONS
Gain Programmable Line Driver
HFC High Speed Data Modems
Interactive CATV Set-Top Boxes
CATV Plant Test Equipment
AD8321
POWER-
DOWN/
SWITCH
INTER
ATTENUATOR CORE
INV
DATA SHIFT REGISTER
DATA LATCH
PWR
AMP
DATEN CLK
REVERSE
AMP
SDATA
General Purpose IF Variable Gain Block
DESCRIPTION
The AD8321 is packaged in a low cost 20-lead SOIC, operates
The AD8321 is a low cost digitally controlled variable gain
from a single +9 V supply, and has an operational temperature
amplifier optimized for coaxial line driving applications such as
range of –40
∞C to +85∞C.
cable modems that are designed to the DOCSIS
* (upstream)
Very Low Output Noise Level
Maintains Constant 75
Output Impedance
Power-Up and Power-Down Condition
No Line Transformer Required
VIN+
Upper Bandwidth: 235 MHz (Min Gain)
VIN–
9V Single Supply Operation
Power-Down Functionality
Supports SPI Interface
Low Cost
VOUT
PD
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.
–40
The AD8321 comprises a digitally controlled variable attenuator
of 0 dB to –53.4 dB, which is preceded by a low noise, fixed
–50
fO =
VIN =
(PIN =
4
137mV
–15dB
2MHz
p-p
m)
(POUT
MAX
= 11d
GAIN)
Bm @
HD3
HD2
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
W 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.
DISTORTION
–
dBc
–60
–70
A key performance and cost advantage of the AD8321 results
from the ability to maintain a constant 75
W output impedance
during power-up and power-down conditions. This eliminates
the need for external 75
W 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
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
under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
–80
–90
0
8
16
24
32
40
48
56
64
72
GAIN CONTROL – Decimal
Figure 1. Harmonic Distortion vs. Gain Control
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
www.analog.com
Fax: 781/461-3113
2005 Analog Devices, Inc. All rights reserved.