a
FEATURES
Broad Range Analog Variable Gain
–2.5 dB to +42.5 dB
3 dB Cutoff Frequency of 500 MHz
Gain Up and Gain Down Modes
Linear-in-dB, Scaled 20 mV/dB
Resistive Ground Referenced Input
Nominal Z
IN
200
On-Chip Square-Law Detector
Single-Supply Operation: 2.7 V to 5.5 V
APPLICATIONS
Cellular Base Station
Broadband Access
Power Amplifier Control Loops
Complete, Linear IF AGC Amplifiers
High-Speed Data I/O
ICOM
INPT
500 MHz, Linear-in-dB VGA
with AGC Detector
AD8367
FUNCTIONAL BLOCK DIAGRAM
VPSI
VPSO
ENBL
AD8367
9-STAGE ATTENUATOR BY 5dB
BIAS
ICOM
DECL
HPFL
g
m
CELLS
SQUARE
LAW
DETECTOR
VOUT
GAUSSIAN INTERPOLATOR
ICOM
OCOM
MODE
GAIN
DETO
GENERAL DESCRIPTION
The AD8367 is a high-performance 45 dB variable gain ampli-
fier with linear-in-dB gain control for use from low frequencies
up to several hundred megahertz. The range, flatness, and accu-
racy of the gain response are achieved using Analog Devices’
X-AMP
®
architecture, the most recent in a series of powerful
proprietary concepts for variable gain applications, which far
surpasses what can be achieved using competing techniques.
The input is applied to a 200
Ω
resistive ladder network, having
nine sections each of 5 dB loss, for a total attenuation of 45 dB.
At maximum gain, the first tap is selected; at progressively lower
gains, the tap moves smoothly and continuously toward higher
attenuation values. The attenuator is followed by a 42.5 dB
fixed gain feedback amplifier—essentially an operational ampli-
fier with a gain bandwidth product of 100 GHz—and is very
linear, even at high frequencies. The output third order intercept is
+20 dBV at 100 MHz (+27 dBm re 200
Ω),
measured at an
output level of 1 V p-p with V
S
= 5 V.
The analog gain-control interface is very simple to use. It is
scaled at 20 mV/dB, and the control voltage, V
GAIN
, runs from
50 mV at –2.5 dB to 950 mV at +42.5 dB. In the inverse-gain
mode of operation, selected by a simple pin-strap, the gain
decreases from +42.5 dB at V
GAIN
= 50 mV to –2.5 dB at
V
GAIN
= 950 mV. This inverse mode is needed in AGC applications,
which are supported by the integrated square-law detector,
whose set point is chosen to level the output to 354 mV rms,
regardless of the waveshape. A single external capacitor sets up
the loop averaging time.
The AD8367 may be powered on or off by a voltage applied to
the ENBL pin. When this voltage is at a logic LO, the total power
dissipation drops to the milliwatt range. For a logic HI, the chip
powers-up rapidly to its normal quiescent current of 26 mA at
25°C. The AD8367 is available in a 14-lead TSSOP package for
the industrial temperature range of –40°C to +85°C.
X-AMP is a registered trademark of Analog Devices, Inc.
REV. 0
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.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
Fax: 781/326-8703
© Analog Devices, Inc., 2001
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