Low Power, 18 MHz Variable Gain Amplifier

Data Sheet

AD8338

Rev. 0

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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

Adjustable gain range

Low noise: 4.5 nV/√Hz

Fully differential signal path

Offset correction (offset null) feature

Adjustable bandwidth

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

RF baseband signal conditioning

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 frequencies from LF to 18 MHz. The

device can also operate using single-ended sources if required.

The basic gain function is linear-in-dB with a nominal gain range

of 0 dB to 80 dB; the nominal gain range corresponds to a control

voltage on the GAIN pin of 0.1 V to 1.1 V. The gain range can be

adjusted up or down via direct access to the internal summing

nodes at the INPD and INMD pins. For example, if a 47 Ω resistor

is applied to the INPD and INMD pins, a gain range of 20 dB to

100 dB is set with an input referred noise level of 1.5 nV√Hz.

The AD8338 includes additional circuits to enable offset correction

and automatic gain control (AGC). DC offset voltages are removed

by the offset correction circuit, which behaves like a high-pass filter.

The high-pass filter corner frequency is set using an external

capacitor. The AGC function varies the gain of the AD8338 to

maintain a constant rms output voltage. A user supplied voltage

controls the target output rms voltage. A user supplied capacitor

to ground at the DETO pin controls the response time of the

AGC circuit.

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.

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 allowing user access

to the internal summing nodes. With a few discrete components,

users can customize the gain, bandwidth, input impedance, and

noise profile of the part 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.

The AD8338 is available in a 3 mm × 3 mm, RoHS compliant,

16-lead LFCSP. It is specified over the industrial temperature range

of −40°C to +85°C.