AD8021 datasheet(23/28 Pages) AD

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ADA4896-2/ADA4897-1

Rev. 0 | Page 23 of 28

MEDICAL ULTRASOUND APPLICATIONS

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

ADA4896-2/

ADA4897-1

TRANSDUCER

ARRAY

MUX/

DEMUX

T/R

SWITCHES

AD9279

AAF

VGA

LNA

ADC

Figure 53. Simplified Ultrasound System Block Diagram

Overview of the Ultrasound System

Medical ultrasound systems are among the most sophisticated

signal processing systems in widespread use today. By transmit-

ting acoustic energy into the body and receiving and processing

the returning reflections, ultrasound systems can generate

images of internal organs and structures, map blood flow

and tissue motion, and provide highly accurate blood velocity

information. Figure 53 shows a simplified block diagram of an

ultrasound system.

The ultrasound system consists of two main operations, the

time gain control (TGC) operation and the continuous wave

(CW) Doppler operation. The AD9279 integrates the essential

components of these two operations into a single IC. It contains

eight channels of a variable gain amplifier (VGA) with a low

noise preamplifier (LNA), an antialiasing filter (AAF), an

analog-to-digital converter (ADC), and an I/Q demodulator

with programmable phase rotation. For detailed information

about how to use the AD9279 in the ultrasound system, refer to

the AD9279 data sheet.

ADA4896-2/ADA4897-1 in the Ultrasound System

The ADA4896-2/ADA4897-1 are used in the CW Doppler path

in the ultrasound application after the I/Q demodulators of

the AD9279. Doppler signals can be typically between 100 Hz

to 100 kHz.The low noise floor, high dynamic range of the

ADA4896-2/ADA4897-1 makes them an excellent choice for

processing weak Doppler signals.

The rail-to-rail output feature and the high output current drive

of the ADA4896-2/ADA4897-1 make them a suitable candidate

for the I-to-V converter, summer, and as a ADC driver.

Figure 54 shows an interconnection block diagram of all eight

channels of the AD9279. Two stages of ADA4896-2 amplifiers

are used. The first stage does an I-to-V conversion and filters

the high frequency content that results from the demodulation

process. The second stage of ADA4896-2 amplifiers is used to

sum the output currents of multiple AD9279, to provide gain,

and to drive the AD7982, an 18-bit SAR ADC.

The output-referred noise of the CW signal path depends on

the LNA gain and the selection of the first stage summing

amplifier and the value of RFILT. To determine the output

referred noise, it is important to know the active low-pass

filter (LPF) values RA, RFILT, and CFILT, as shown as Figure 54.

Typical filter values for all eight channels of a single AD9279

are 100 Ω for RA, 500 Ω for RFILT, and 2.0 nF for CFILT; these

values implement a 100 kHz single-pole LPF.

The gain of the I-to-V converter can be increased by increasing

the filter resistor, RFILT. To keep the corner frequency the same,

decrease the filter capacitor, CFILT, by the same factor. The factor

limiting the magnitude of the gain is the output swing and drive

capability of the op amp selected for the I-to-V converter, in

this example, the ADA4896-2/ADA4897-1. Because any

amplifier has limited drive capability, there is a finite number of

channels that can be summed.

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