Low Power, 18 MHz Variable Gain Amplifier
AD8338
Data Sheet
FEATURES
FUNCTIONAL BLOCK DIAGRAM
VBAT OFSN VREF
Voltage controlled gain range of 0 dB to 80 dB
3 mA supply current at gain of 40 dB
Low frequency (LF) to 18 MHz operation
Supply range: 3.0 V to 5.0 V
Low noise: 4.5 nV/√Hz at 80 dB gain
Fully differential signal path
Offset correction (offset null) feature
Internal 1.5 V reference
16-lead LFCSP
OFFSET NULL
FBKP
AD8338
VGA CORE
+
INPR
+
OUTP
OUTM
OUTPUT
STAGE
0dB
–
–
INMR
INPD
INMD
0dB TO 80dB
VREF
FBKM
Automatic gain control feature
Wide gain range for high dynamic range signals
AUTOMATIC
GAIN
CONTROL
GAIN INTERFACE
APPLICATIONS
Front end for inductive telemetry systems
Ultrasonic signal receivers
Signal compression for driving an ADC
AGC amplifiers
COMM
MODE GAIN
DETO
VAGC
Figure 1.
GENERAL DESCRIPTION
100
The AD8338 is a variable gain amplifier (VGA) for applications
that require a fully differential signal path, low power, low noise,
and a well-defined gain over frequency. Although the inputs are
differential, the device can also be driven with a single-ended
source if required.
V
= 1.1V
GAIN
80
60
V
V
V
V
V
V
V
V
= 1.0V
= 0.9V
= 0.8V
= 0.7V
= 0.6V
= 0.5V
= 0.4V
= 0.3V
GAIN
GAIN
GAIN
GAIN
GAIN
GAIN
GAIN
GAIN
40
The basic gain function is linear-in-dB and is controlled by the
voltage applied to Pin GAIN. The nominal gain range spans
from 0 dB to 80 dB for control voltages between 0.1 V to 1.1 V
with a slope of 12.5 mV/dB. The nominal gain range can be
shifted up or down via direct access to Pin INPD and Pin INMD,
the current inputs of the VGA. For example, driving the INPD
and INMD pins with 50 Ω resistors shifts the gain range up by
20 dB, that is, 20 dB to 100 dB, and lowers the input referred
noise of the device to 1.5 nV/√Hz. Additionally, the gain slope
can be inverted via logic Pin MODE.
20
V
V
= 0.2V
= 0.1V
GAIN
GAIN
0
–20
–40
10k
100k
1M
FREQUENCY (Hz)
10M
100M
Figure 2. Gain vs. Frequency
The AD8338 offers additional versatility by providing access to
the internal summing nodes of the VGA core and the output
amplifiers. With the addition of a few external passive components,
users can customize the gain, bandwidth, input impedance, and
noise profile of the device to fit their application.
The AD8338 includes additional circuit blocks to enable input
offset correction and automatic gain control (AGC). DC offset
voltages are removed by the offset correction circuit, which
behaves like a high-pass filter whose corner is set with an external
capacitor. The AGC function varies the gain of the AD8338 to
maintain a constant rms output voltage. An externally applied
voltage to Pin VAGC with respect to the voltage at Pin VREF
sets the output rms amplitude. A capacitor from Pin DETO to
ground sets the response time of the AGC circuit.
The AD8338 uses a single-supply voltage of 3.0 V to 5.0 V and
is very power efficient, consuming as little as 3 mA quiescent
current at mid gain. The AD8338 is available in a 3 mm × 3 mm,
RoHS-compliant, 16-lead LFCSP and is specified over the
industrial temperature range of −40°C to +85°C.
Rev. B
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Trademarks andregisteredtrademarks are the property of their respective owners.
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Tel: 781.329.4700 ©2013–2016 Analog Devices, Inc. All rights reserved.
Technical Support
www.analog.com
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