a
FEATURES
“Linear in dB” Gain Control
Pin Programmable Gain Ranges
–11 dB to +31 dB with 90 MHz Bandwidth
+9 dB to +51 dB with 9 MHz Bandwidth
Any Intermediate Range, e.g., –1 dB to +41 dB with
30 MHz Bandwidth
Bandwidth Independent of Variable Gain
1.3 nV/√Hz Input Noise Spectral Density
0.5 dB Typical Gain Accuracy
MIL-STD-883 Compliant and DESC Versions Available
APPLICATIONS
RF/IF AGC Amplifier
Video Gain Control
A/D Range Extension
Signal Measurement
PRODUCT DESCRIPTION
Low Noise, 90 MHz
Variable-Gain Amplifier
AD603*
1 V to span the central 40 dB of the gain range. An over- and
under-range of 1 dB is provided whatever the selected range. The
gain-control response time is less than 1
µs
for a 40 dB change.
The differential gain-control interface allows the use of either
differential or single-ended positive or negative control voltages.
Several of these amplifiers may be cascaded and their gain-con-
trol gains offset to optimize the system S/N ratio.
The AD603 can drive a load impedance as low as 100
Ω
with
low distortion. For a 500
Ω
load in shunt with 5 pF, the total
harmonic distortion for a
±
1 V sinusoidal output at 10 MHz is
typically –60 dBc. The peak specified output is
±
2.5 V mini-
mum into a 500
Ω
load, or
±
1 V into a 100
Ω
load.
The AD603 uses a proprietary circuit topology—the X-AMP™.
The X-AMP comprises a variable attenuator of 0 dB to
–42.14 dB followed by a fixed-gain amplifier. Because of the
attenuator, the amplifier never has to cope with large inputs and
can use negative feedback to define its (fixed) gain and dynamic
performance. The attenuator has an input resistance of 100
Ω,
laser trimmed to
±
3%, and comprises a seven-stage R-2R ladder
network, resulting in an attenuation between tap points of
6.021 dB. A proprietary interpolation technique provides a
continuous gain-control function which is linear in dB.
The AD603A is specified for operation from –40°C to +85°C
and is available in both 8-lead SOIC (R) and 8-lead ceramic
DIP (Q). The AD603S is specified for operation from –55°C to
+125°C and is available in an 8-lead ceramic DIP (Q). The
AD603 is also available under DESC SMD 5962-94572.
The AD603 is a low noise, voltage-controlled amplifier for use
in RF and IF AGC systems. It provides accurate, pin selectable
gains of –11 dB to +31 dB with a bandwidth of 90 MHz or
+9 dB to +51 dB with a bandwidth of 9 MHz. Any intermediate
gain range may be arranged using one external resistor. The
input referred noise spectral density is only 1.3 nV/√Hz and power
consumption is 125 mW at the recommended
±5
V supplies.
The decibel gain is “linear in dB,” accurately calibrated, and
stable over temperature and supply. The gain is controlled at a
high impedance (50 MΩ), low bias (200 nA) differential input;
the scaling is 25 mV/dB, requiring a gain-control voltage of only
FUNCTIONAL BLOCK DIAGRAM
VPOS
VNEG
GPOS
SCALING
REFERENCE
PRECISION PASSIVE
INPUT ATTENUATOR
FIXED GAIN
AMPLIFIER
V
OUT
V
G
GNEG
GAIN
CONTROL
INTERFACE
AD603
6.44k *
FDBK
694 *
0dB
VINP
–6.02dB –12.04dB –18.06dB –24.08dB –30.1dB –36.12dB –42.14dB
R
2R
R
2R
R
2R
R
2R
R
2R
R
2R
R
R
*NORMAL VALUES
20 *
COMM
R = 2R LADDER NETWORK
*Patented.
X-AMP is a trademark of Analog Devices, Inc.
REV. C
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
which 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
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 2000
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