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 MW), low bias (200 nA) differential input;
the scaling is 25 mV/dB, requiring a gain-control voltage of only

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 ms 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-control
gains offset to optimize the system S/N ratio.
The AD603 can drive a load impedance as low as 100 W with
low distortion. For a 500 W 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 minimum
into a 500 W load, or ±1 V into a 100 W 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 W,
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.

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
60.5 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