Datasheet

AD8335 Data Sheet
Rev. B | Page 20 of 28
APPLICATIONS INFORMATION
ULTRASOUND
The primary application for the AD8335 is medical ultrasound.
Figure 57 shows a simplified block diagram of an ultrasound
system. The most 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 (for example, 32 to 512) of channels, the
lowest power at the lowest possible noise is of key importance.
Most modern machines use digital beamforming. In this technique,
the signal is converted to digital format immediately following
the TGC amplifier; beamforming is done digitally.
Typical ADC resolution in general purpose machines is 10 bits
with sampling rates greater than 40 MSPS, and high-end systems
use 12 bits.
Power consumption and low cost are of primary importance in
low-end and portable ultrasound machines, and the AD8335 is
designed for these criteria.
For additional information regarding ultrasound systems, refer
to “How Ultrasound System Considerations Influence Front-End
Component Choice”, Analog Dialogue, Vol. 36, No. 3, May–July
2003. (www.analog.com/library/analogDialogue/archives/36-
03/ultrasound/index.html)
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
LNAs
TRANSDUCER
ARRAY
128, 256 ETC.
ELEMENTS
BIDIRECTIONAL
CABLE
HV
MUX/
DEMUX
T/R
SWITCHES
TX HV AMPs
MULTICHANNEL
TGC USES MANY VGAs
TGC
TIME GAIN COMPENSATION
04976-053
VGAs
AD8335
Figure 57. Simplified Ultrasound System Block Diagram