Low Power, 18 MHz Variable Gain Amplifier

Data Sheet

AD8338

Rev. B

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FEATURES

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

Automatic gain control feature

Wide gain range for high dynamic range signals

APPLICATIONS

Front end for inductive telemetry systems

Ultrasonic signal receivers

Signal compression for driving an ADC

AGC amplifiers

FUNCTIONAL BLOCK DIAGRAM

INPR

INPD

INMD

INMR

MODE

COMM

GAIN

FBKM

OUTP

OUTM

DETO

VAGC

FBKP

+

–

+

–

VREF

VGA CORE

0dB TO 80dB

OFFSET NULL

OUTPUT

STAGE

0dB

OFSN

VREF

VBAT

AUTOMATIC

GAIN

CONTROL

GAIN INTERFACE

AD8338

Figure 1.

GENERAL DESCRIPTION

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.

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.

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.

100

–40

–20

0

20

40

60

80

10k

100M

10M

1M

100k

FREQUENCY (Hz)

VGAIN = 0.1V

VGAIN = 0.2V

VGAIN = 0.3V

VGAIN = 0.4V

VGAIN = 0.5V

VGAIN = 0.6V

VGAIN = 0.7V

VGAIN = 0.8V

VGAIN = 0.9V

VGAIN = 1.0V

VGAIN = 1.1V

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 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.