Ultralow Noise VGAs with
Preamplifier and Programmable RIN
AD8331/AD8332/AD8334
Rev. G
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FEATURES
Ultralow noise preamplifier (preamp)
Voltage noise = 0.74 nV/√Hz
Current noise = 2.5 pA/√Hz
3 dB bandwidth
Low power
Wide gain range with programmable postamp
4.5 dB to +43.5 dB in LO gain mode
7.5 dB to 55.5 dB in HI gain mode
Low output-referred noise: 48 nV/√Hz typical
Active input impedance matching
Optimized for 10-bit/12-bit ADCs
Selectable output clamping level
Single 5 V supply operation
AD8332 and
AD8334 available in lead frame chip scale package
APPLICATIONS
Ultrasound and sonar time-gain controls
High performance automatic gain control (AGC) systems
I/Q signal processing
High speed, dual ADC drivers
GENERAL DESCRIPTION
channel, ultralow noise linear-in-dB, variable gain amplifiers
(VGAs). Optimized for ultrasound systems, they are usable as a
low noise variable gain element at frequencies up to 120 MHz.
Included in each channel are an ultralow noise preamp (LNA),
an X-AMP VGA with 48 dB of gain range, and a selectable gain
postamp with adjustable output limiting. The LNA gain is 19 dB
with a single-ended input and differential outputs. Using a single
resistor, the LNA input impedance can be adjusted to match a
signal source without compromising noise performance.
The 48 dB gain range of the VGA makes these devices suitable
for a variety of applications. Excellent bandwidth uniformity is
maintained across the entire range. The gain control interface
provides precise linear-in-dB scaling of 50 dB/V for control
voltages between 40 mV and 1 V. Factory trim ensures excellent
part-to-part and channel-to-channel gain matching.
FUNCTIONAL BLOCK DIAGRAM
0
319
9-
0
01
VOL
VOH
VMID
LNA
48dB
ATTENUATOR
ENB
INH
LMD
VIN
VIP
LOP
LON
GAIN
AD8331/AD8332/AD8334
+
–
CLAMP
RCLMP
HILO
VCM
3.5dB OR 15.5dB
19dB
PA
VCM
BIAS
VGA BIAS AND
INTERPOLATOR
GAIN
CONTROL
INTERFACE
21dB
Figure 1. Signal Path Block Diagram
60
50
40
30
20
10
0
–10
100k
1M
10M
100M
1G
GA
IN
(
d
B
)
FREQUENCY (Hz)
03
19
9-
0
02
VGAIN = 1V
VGAIN = 0.8V
VGAIN = 0.6V
VGAIN = 0.4V
VGAIN = 0.2V
VGAIN = 0V
HI GAIN
MODE
Figure 2. Frequency Response vs. Gain
Differential signal paths result in superb second- and third-
order distortion performance and low crosstalk.
The low output-referred noise of the VGA is advantageous in
driving high speed differential ADCs. The gain of the postamp
can be pin selected to 3.5 dB or 15.5 dB to optimize gain range
and output noise for 12-bit or 10-bit converter applications. The
output can be limited to a user-selected clamping level, preventing
input overload to a subsequent ADC. An external resistor adjusts
the clamping level.
The operating temperature range is 40°C to +85°C. The
AD8331 is available in a 20-lead QSOP package, the
AD8332 is
available in 28-lead TSSOP and 32-lead LFCSP packages, and
the
AD8334 is available in a 64-lead LFCSP package.