REV. C

a

AD605

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Dual, Low Noise, Single-Supply

Variable Gain Amplifier

FUNCTIONAL BLOCK DIAGRAM

PRECISION PASSIVE

INPUT ATTENUATOR

FIXED GAIN

AMPLIFIER

+34.4dB

DIFFERENTIAL

ATTENUATOR

0 TO –48.4dB

OUT

VOCM

VGN

VREF

+IN

–IN

GAIN

CONTROL

AND

SCALING

FBK

AD605

FEATURES

Two Independent Linear-in-dB Channels

Input Noise at Maximum Gain: 1.8 nV/

√Hz, 2.7 pA/√Hz

Bandwidth: 40 MHz (–3 dB)

Differential Input

Absolute Gain Range Programmable:

–14 dB to +34 dB (FBK Shorted to OUT) Through

0 dB to +48 dB (FBK Open)

Variable Gain Scaling: 20 dB/V Through 40 dB/V

Stable Gain with Temperature and Supply Variations

Single-Ended Unipolar Gain Control

Output Common-Mode Independently Set

Power Shutdown at Lower End of Gain Control

Single 5 V Supply

Low Power: 90 mW/Channel

Drives A/D Converters Directly

APPLICATIONS

Ultrasound and Sonar Time-Gain Control

High Performance AGC Systems

Signal Measurement

GENERAL DESCRIPTION

The AD605 is a low noise, accurate, dual channel, linear-in-dB

variable gain amplifier, which is optimized for any application

requiring high performance, wide bandwidth variable gain con-

trol. Operating from a single 5 V supply, the AD605 provides

differential inputs and unipolar gain control for ease of use.

Added flexibility is achieved with a user-determined gain range

and an external reference input which provides user-determined

gain scaling (dB/V).

The high performance linear-in-dB response of the AD605 is

achieved with the differential input, single supply, exponential

amplifier (DSX-AMP) architecture. Each of the DSX-AMPs

comprise a variable attenuator of 0 dB to –48.4 dB followed by

a high speed fixed gain amplifier. The attenuator is based on a

7-stage R-1.5R ladder network. The attenuation between tap

points is 6.908 dB, and 48.360 dB for the entire ladder network.

The DSX-AMP architecture results in 1.8 nV/

√Hz input noise

spectral density and will accept a

± 2.0 V input signal when

VOCM is biased at VP/2.

Each independent channel of the AD605 provides a gain range

of 48 dB which can be optimized for the application. Gain

ranges between –14 dB to +34 dB and 0 dB to +48 dB can be

selected by a single resistor between pins FBK and OUT. The

lower and upper gain ranges are determined by shorting pin

FBK to OUT, or leaving pin FBK unconnected, respectively.

The two channels of the AD605 can be cascaded to provide 96

dB of very accurate gain range in a monolithic package.

The gain control interface provides an input resistance of approxi-

mately 2 M

Ω and scale factors from 20 dB/V to 30 dB/V for a

VREF input voltage of 2.5 V to 1.67 V, respectively. Note that

scale factors up to 40 dB/V are achievable with reduced accu-

racy for scales above 30 dB/V. The gain scales linearly in dB

with control voltages (VGN) of 0.4 V to 2.4 V for the 20 dB/V

scale and 0.20 V to 1.20 V for the 40 dB/V scale. When VGN

is <50 mV the amplifier is powered down to draw 1.9 mA.

Under normal operation, the quiescent supply current of each

amplifier channel is only 18 mA.

The AD605 is available in 16-lead PDIP and SOIC, and is

guaranteed for operation over the –40

°C to +85°C tempera-

ture range.