REV. 0

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a

Dual, Ultralow Noise

Variable Gain Amplifier

AD604

FEATURES

Ultralow Input Noise at Maximum Gain:

0.80 nV/

√Hz, 3.0 pA/√Hz

Two Independent Linear-in-dB Channels

Absolute Gain Range per Channel Programmable:

0 dB to +48 dB (Preamp Gain = +14 dB), through

+6 dB to +54 dB (Preamp Gain = +20 dB)

1.0 dB Gain Accuracy

Bandwidth: 40 MHz (–3 dB)

300 k

Input Resistance

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

Stable Gain with Temperature and Supply Variations

Single-Ended Unipolar Gain Control

Power Shutdown at Lower End of Gain Control

Can Drive A/D Converters Directly

APPLICATIONS

Ultrasound and Sonar Time-Gain Control

High Performance AGC Systems

Signal Measurement

PRODUCT DESCRIPTION

The AD604 is an ultralow noise, very accurate, dual channel,

linear-in-dB variable gain amplifier (VGA) optimized for time-

based variable gain control in ultrasound applications; however

it will support any application requiring low noise, wide bandwidth

variable gain control. Each channel of the AD604 provides a

300 k

Ω input resistance and unipolar gain control for ease of

use. User determined gain ranges, gain scaling (dB/V) and dc

level shifting of output further optimize application performance.

Each channel of the AD604 utilizes a high performance pre-

amplifier that provides an input referred noise voltage of

0.8 nV/

√Hz. The very accurate linear-in-dB response of the

AD604 is achieved with the differential input exponential amplifier

(DSX-AMP) architecture. Each of the DSX-AMPs comprise a

variable attenuator of 0 dB to 48.36 dB followed by a high speed

fixed gain amplifier. The attenuator is based on a seven stage

R-1.5R ladder network. The attenuation between tap points

is 6.908 dB and 48.36 dB for the ladder network.

Each independent channel of the AD604 provides a gain range

of 48 dB which can be optimized for the application by program-

ming the preamplifier with a single external resistor in the

preamp feedback path. The linear-in-dB gain response of the

AD604 can be described by the equation: G (dB) = (Gain

Scaling (dB/V )

× VGN (V )) + (Preamp Gain (dB) – 19 dB).

Preamplifier gains between 5 and 10 (+14 dB and +20 dB)

FUNCTIONAL BLOCK DIAGRAM

OUT

VOCM

PAO

PAI

DIFFERENTIAL

ATTENUATOR

+DSX

–DSX

VGN

VREF

R-1.5R

LADDER NETWORK

0 TO –48.4dB

PROGRAMMABLE

ULTRALOW NOISE

PREAMPLIFIER

G = 14–20dB

PRECISION PASSIVE

INPUT ATTENUATOR

FIXED GAIN

AMPLIFIER

+34.4dB

AFA

GAIN CONTROL

AND SCALING

provide overall gain ranges per channel of 0 dB through +48 dB

and +6 dB through +54 dB. The two channels of the AD604

can be cascaded to provide greater levels of gain range by bypass-

ing the 2nd channel’s preamplifier. However, in multiple channel

systems, cascading the AD604 with other devices in the AD60x

VGA family, which do not include a preamplifier may provide

a more efficient solution. The AD604 provides access to the

output of the preamplifier allowing for external filtering be-

tween the preamplifier and the differential attenuator stage.

The gain control interface provides an input resistance of

approximately 2 M

Ω and scale factors from 20 dB/V to

ively. Note that scale factors up to 40 dB/V are achievable

with reduced accuracy for scales above 30 dB/V. The gain scales

linear-in-dB with control voltages of 0.4 V to 2.4 V with the

20 dB/V scale. Below and above this gain control range, the gain

begins to deviate from the ideal linear-in-dB control law. The

gain control region below 0.1 V is not used for gain control. In

fact when the gain control voltage is <50 mV the amplifier

channel is powered down to 1.9 mA.

The AD604 is available in a 24-pin plastic SSOP, SOIC and DIP,

and is guaranteed for operation over the –40

°C to +85°C

temperature range.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 617/329-4700

World Wide Web Site: http://www.analog.com

Fax: 617/326-8703

© Analog Devices, Inc., 1996