Analog Devices AD604 Car Amplifier User Manual

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Dual, Ultralow Noise

Variable Gain Amplifier

AD604

Rev. F

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responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other

rights of third parties that may result from its use. Specifications subject to change without notice. No

license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

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

Tel: 781.329.4700 www.analog.com

FEATURES

Ultralow input noise at maximum gain

0.80 nV/√Hz, 3.0 pA/√Hz

2 independent linear-in-dB channels

Absolute gain range per channel programmable

0 dB to 48 dB (preamplifier gain = 14 dB) through 6 dB to

54 dB (preamplifier gain = 20 dB)

±1.0 dB gain accuracy

Bandwidth: 40 MHz (−3 dB)

Input resistance: 300 kΩ

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

Drive ADCs directly

APPLICATIONS

Ultrasound and sonar time-gain controls

High performance AGC systems

Signal measurement

FUNCTIONAL BLOCK DIAGRAM

OUTx

VOCM

PAOx

PAIx

+DSXx

–

DSXx

GNx

VREF

AFA

FIXED GAIN

AMPLIFIER

34.4dB

PRECISION PASSIVE

INPUT ATTENUATOR

PROGRAMMABLE

ULTRALOW NOISE

PREAMPLIFIER

G = 14dB TO 20dB

DIFFERENTIAL

ATTENUATOR

R-1.5R

LADDER NETWORK

0dB TO –48.4dB

GAIN CONTROL

AND SCALING

00540-001

Figure 1.

GENERAL 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 supports any application requiring low noise, wide bandwidth,

variable gain control. Each channel of the AD604 provides a

300 kΩ 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 performance.

Each channel of the AD604 uses a high performance

preamplifier 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 DSX-AMP comprises

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

speed fixed-gain amplifier. The attenuator is a 7-stage

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

6.908 dB and 48.36 dB for the ladder network.

The equation for the linear-in-dB gain response is

G (dB) =

(Gain Scaling (dB/V) × VGN (V)) + (Preamp Gain (dB) – 19 dB)

Preamplifier gains between 5 and 10 (14 dB and 20 dB) 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 bypassing the preamplifier

of the second channel. However, in multiple channel systems,

cascading the AD604 with other devices in the AD60x VGA

family that 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 between the

preamplifier and the differential attenuator stage.

Note that scale factors up to 40 dB/V are achievable with reduced

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

decibels 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. When

the gain control voltage is <50 mV, the amplifier channel is

powered down to 1.9 mA.

The AD604 is available in 24-lead SSOP, SOIC, and PDIP

packages and is guaranteed for operation over the −40°C to

+85°C temperature range.

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Analog Devices AD604 Car Amplifier User Manual