Datasheet

AD9277
Rev. 0 | Page 21 of 48
THEORY OF OPERATION
ULTRASOUND
The primary application for the AD9277 is medical ultrasound.
Figure 45 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 14 bits with up to 50 MSPS sampling
satisfies the requirements of both general-purpose and high
end systems.
Power conservation and low cost are two of the most important
factors in low end and portable ultrasound machines, and the
AD9277 is designed to meet 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.
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, ETC.,
ELEMENTS
BIDIRECTIONAL
CABLE
HV
MUX/
DEMUX
T/R
SWITCHES
Tx HV AMPLIFIERS
MULTICHANNELS
AAF
VGALNA
ADC
08181-039
Figure 45. Simplified Ultrasound System Block Diagram