Datasheet

AD9271
Rev. B | Page 20 of 60
THEORY OF OPERATION
ULTRASOUND
The primary application for the AD9271 is medical ultrasound.
Figure 39 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 beam-forming techniques requiring large binary-
weighted numbers (for example, 32 to 512) of channels, the
lowest power at the lowest possible noise is of key importance.
Most modern machines use digital beam forming. 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.
Power consumption and low cost are of primary importance in
low-end and portable ultrasound machines, and the AD9271 is
designed for these criteria.
For additional information regarding ultrasound systems, refer
to “
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.
Tx H
V
BEAM FORMER
CENTRAL CONTROL
Rx BEAM FORMER
(B AND F MODES)
COLOR
DOPPLER (PW)
PROCESSING
(F MODE)
IMAGE AND
MOTION
PROCESSING
(B MODE)
SPECTRAL
DOPPLER
PROCESSING
MODE
DISPLAY
AUDIO
OUTPUT
Tx BEAM FORMER
CW (ANALOG)
BEAM FORMER
AMPs
MULTICHANNELS
TRANSDUCER
ARRAY
128, 256, ETC.,
ELEMENTS
BIDIRECTIONAL
CABLE
HV
MUX/
DEMUX
T/R
SWITCHES
06304-077
AD9271
AAF
VGA
ADC
LNA
CW
Figure 39. Simplified Ultrasound System Block Diagram