Dual, Ultralow Noise
Variable Gain Amplifier
AD604
Rev. E
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FEATURES
Ultralow input noise at maximum gain
0.80 nV/√Hz, 3.0 pA/√Hz
2 independent linear-in-dB channels
Absolute gain range per channel programmable
0 dB to 48 dB (preamplifier gain = 14 dB) through 6 dB to
54 dB (preamplifier gain = 20 dB)
±1.0 dB gain accuracy
Bandwidth: 40 MHz (−3 dB)
Input resistance: 300 kΩ
Variable gain scaling: 20 dB/V through 40 dB/V
Stable gain with temperature and supply variations
Single-ended unipolar gain control
Power shutdown at lower end of gain control
Drive ADCs directly
APPLICATIONS
Ultrasound and sonar time-gain controls
High performance AGC systems
Signal measurement
FUNCTIONAL BLOCK DIAGRAM
OUTx
VOCM
PAOx
PAIx
+DSXx
–DSXx
VGNx
VREF
AFA
FIXED GAIN
AMPLIFIER
34.4dB
PRECISION PASSIVE
INPUT ATTENUATOR
PROGRAMMABLE
ULTRALOW NOISE
PREAMPLIFIER
G = 14dB TO 20dB
DIFFERENTIAL
ATTENUATOR
R-1.5R
LADDER NETWORK
0dB TO –48.4dB
GAIN CONTROL
AND SCALING
00
540
-00
1
Figure 1.
GENERAL DESCRIPTION
The AD604 is an ultralow noise, very accurate, dual-channel,
linear-in-dB variable gain amplifier (VGA) optimized for time-
based variable gain control in ultrasound applications; however,
it supports any application requiring low noise, wide bandwidth,
variable gain control. Each channel of the AD604 provides a
300 kΩ input resistance and unipolar gain control for ease of
use. User-determined gain ranges, gain scaling (dB/V), and dc
level shifting of output further optimize performance.
Each channel of the AD604 uses a high performance
preamplifier that provides an input-referred noise voltage of
0.8 nV/√Hz. The very accurate linear-in-dB response of the
AD604 is achieved with the differential input exponential
amplifier (DSX-AMP) architecture. Each DSX-AMP comprises
a variable attenuator of 0 dB to 48.36 dB followed by a high
speed fixed-gain amplifier. The attenuator is a 7-stage
R-1.5R ladder network. The attenuation between tap points is
6.908 dB and 48.36 dB for the ladder network.
The equation for the linear-in-dB gain response is
G
(dB) =
(
Gain Scaling
(dB/V) ×
VGN
(V)) + (
Preamp Gain
(dB) – 19 dB)
Preamplifier gains between 5 and 10 (14 dB and 20 dB) provide
overall gain ranges per channel of 0 dB through 48 dB and 6 dB
through 54 dB. The two channels of the AD604 can be cascaded
to provide greater levels of gain range by bypassing the preamplifier
of the second channel. However, in multiple channel systems,
cascading the AD604 with other devices in the AD60x VGA
family that do not include a preamplifier may provide a more
efficient solution. The AD604 provides access to the output of
the preamplifier, allowing for external filtering between the
preamplifier and the differential attenuator stage.
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
decibels with control voltages of 0.4 V to 2.4 V with the 20 dB/V
scale. Below and above this gain control range, the gain begins
to deviate from the ideal linear-in-dB control law. The gain
control region below 0.1 V is not used for gain control. When
the gain control voltage is <50 mV, the amplifier channel is
powered down to 1.9 mA.
The AD604 is available in 24-lead SSOP, SOIC, and PDIP
packages and is guaranteed for operation over the −40°C to
+85°C temperature range.
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