AD9278 datasheet(20/45 Pages) AD

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Data Sheet

AD9278

Rev. A | Page 19 of 44

ULTRASOUND THEORY OF OPERATION

BEAMFORMER

CENTRAL CONTROL

Rx BEAMFORMER

(B AND F MODES)

COLOR

DOPPLER (PW)

PROCESSING

(F MODE)

IMAGE AND

MOTION

PROCESSING

(B MODE)

SPECTRAL

DOPPLER

PROCESSING

MODE

DISPLAY

AUDIO

OUTPUT

Tx BEAMFORMER

CW (ANALOG)

BEAMFORMER

TRANSDUCER

ARRAY

128, 256, ...

ELEMENTS

BIDIRECTIONAL

CABLE

MUX/

DEMUX

T/R

SWITCHES

Tx HV AMPLIFIERS

MULTICHANNELS

AD9278

AAF

VGA

LNA

ADC

Figure 37. Simplified Ultrasound System Block Diagram

The primary application for the AD9278 is medical ultrasound.

Figure 37 shows a simplified block diagram of an ultrasound

system. A critical function of an ultrasound system is the time

gain control (TGC) compensation for physiological signal

attenuation. Because the attenuation of ultrasound signals is

exponential with respect to distance (time), a linear-in-dB VGA

is the optimal solution.

Key requirements in an ultrasound signal chain are very low

noise, active input termination, fast overload recovery, low power,

and differential drive to an ADC. Because ultrasound machines

use beamforming techniques requiring large binary-weighted

numbers of channels (for example, 32 to 512), using the lowest

power at the lowest possible noise is of chief importance.

Most modern ultrasound machines use digital beamforming.

In this technique, the signal is converted to digital format

immediately following the TGC amplifier, and then beam-

forming is accomplished digitally.

The ADC resolution of 12 bits with up to 50 MSPS sampling

satisfies the requirements of both general-purpose and high

end systems. The power dissipation of the ADC scales with

programmable speed modes for optimum power performance

depending on system architecture.

Power conservation, high performance, and low cost are three

of the most important factors in low end and portable ultra-

sound machines, and the AD9278 is designed to meet these

criteria.

For additional information regarding ultrasound systems, see

“How Ultrasound System Considerations Influence Front-End

Component Choice,” Analog Dialogue, Volume 36, Number 3,

May–July 2002, and “The AD9271—A Revolutionary Solution

for Portable Ultrasound,” Analog Dialogue, Volume 41, Number 7,

July 2007.

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