Dual, Low Noise, Single-Supply
Variable Gain Amplifier
AD605
Rev. F
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FEATURES
2 independent linear-in-dB channels
Input noise at maximum gain: 1.8 nV/√Hz, 2.7 pA/√Hz
Bandwidth: 40 MHz (−3 dB)
Differential input
Absolute gain range programmable
−14 dB to +34 dB (FBK shorted to OUT) through
0 dB to 48 dB (FBK open)
Variable gain scaling: 20 dB/V through 40 dB/V
Stable gain with temperature and supply variations
Single-ended unipolar gain control
Output common mode independently set
Power shutdown at lower end of gain control
Single 5 V supply
Low power: 90 mW/channel
Drives ADCs directly
APPLICATIONS
Ultrasound and sonar time-gain controls
High performance AGC systems
Signal measurement
FUNCTIONAL BLOCK DIAGRAM
PRECISION PASSIVE
INPUT ATTENUATOR
FIXED GAIN
AMPLIFIER
+34.4dB
DIFFERENTIAL
ATTENUATOR
0 TO –48.4dB
OUT
VOCM
VGN
VREF
+IN
–IN
GAIN
CONTROL
AND
SCALING
FBK
AD605
0541-001
Figure 1.
GENERAL DESCRIPTION
The AD605 is a low noise, accurate, dual-channel, linear-in-dB
variable gain amplifier (VGA), optimized for any application
requiring high performance, wide bandwidth variable gain
control. Operating from a single 5 V supply, the AD605 provides
differential inputs and unipolar gain control for ease of use.
Added flexibility is achieved with a user-determined gain range
and an external reference input that provide user-determined
gain scaling (dB/V).
The high performance linear-in-dB response of the AD605 is
achieved with the differential input, single-supply, exponential
amplifier (DSX-AMP) architecture. Each of the DSX-AMPs
comprises a variable attenuator of 0 dB to −48.4 dB followed by
a high speed, fixed-gain amplifier. The attenuator is based on a
7-stage R-1.5R ladder network. The attenuation between tap
points is 6.908 dB, and 48.360 dB for the entire ladder network.
The DSX-AMP architecture results in 1.8 nV/√Hz input noise
spectral density and accepts a ±2.0 V input signal when VOCM
is biased at VP/2.
Each independent channel of the AD605 provides a gain range
of 48 dB that can be optimized for the application. Gain ranges
between −14 dB to +34 dB and 0 dB to +48 dB can be selected
by a single resistor between Pin FBK and Pin OUT. The lower
and upper gain ranges are determined by shorting Pin FBK to
Pin OUT or leaving Pin FBK unconnected, respectively. The
two channels of the AD605 can be cascaded to provide 96 dB
of very accurate gain range in a monolithic package.
The gain control interface provides an input resistance of
approximately 2 MΩ and scale factors from 20 dB/V to 30 dB/V
for a VREF input voltage of 2.5 V to 1.67 V, respectively. Note
that scale factors up to 40 dB/V are achievable with reduced
accuracy for scales above 30 dB/V. The gain scales linearly in dB
with control voltages (VGN) of 0.4 V to 2.4 V for the 20 dB/V
scale and 0.20 V to 1.20 V for the 40 dB/V scale. When VGN is
<50 mV, the amplifier is powered down to draw 1.9 mA. Under
normal operation, the quiescent supply current of each amplifier
channel is only 18 mA.
The AD605 is available in a 16-lead PDIP and a 16-lead SOIC_N
package and is guaranteed for operation over the −40°C to +85°C
temperature range.
