An emerging business philosophy in
semiconductor design says that the
way to prosper in the new global
marketplace is to use your engineering
skills to design your customers’ products
for them—or at least the “hard parts.”
One corollary of this is that you have to
keep beating your own previous personal-best
benchmarks over and over again at the same
old 18-month cycles, not just at some component
level, but at the subsystem level. The
reward is that you get to keep doing it. If you
do it right and pick your markets wisely, you
can also wind up owning a key component to
a ubiquitous technology.
One such potentially ubiquitous technology
is diagnostic ultrasound, which is poised to
move far beyond the baby-picture business.
As a diagnostic tool for cardiology, for assessing
the condition of accident victims at the
scene, and for patient monitoring in the operating
theater, ultrasound is becoming indispensible
across a spectrum of medical specialties
beyond obstetrics. In recognition of this, companies
in Europe, the United States, and Asia
are pursuing the technology across a range of
resolutions, form factors, power requirements,
and price points.
So maybe it pays to gamble on ultrasound in
much the same way that companies used to gamble
on set-top boxes, personal media players, cars, or
appliances. Maybe the margins offset the volumes.
Maybe if we’re talking about equipping emergency medical technicians, the volumes aren’t so small. Not to
mention, there are at least 128 analog transducer channels
even in today’s simplest ultrasound probe head. That seems
to double in every generation.
With those points in mind, last year, Analog Devices and
Texas Instruments introduced eight-channel analog front
ends (AFEs) for ultrasound equipment. ADI started shipping
the AD9271 in April of 2007, and TI announced the
AFE5805, which began sampling in April of this year (see
“Monolithic Ultrasound AFEs Usurp Multiple Chips In New
Designs” at www.electronicdesign.com, ED Online 18660).
Analog Devices has now updated the AD9271 with two
new eight-channel chips that are almost pin-compatible but
offer greater flexibility in terms of application targeting. To
aid in using this flexibility, there is also an SPI-bus (serial port
interface) interface and GUI for tweaking internal registers via
that interface.
The basic architecture carries through (Fig. 1). There are
still eight channels, each comprising a low-noise amplifier
(LNA), a variable-gain amp (VGA) to adjust the gain of
the channel over time, an anti-aliasing filter (AAF), a 12-bit
analog-to-digital converter (ADC), and a serial low-voltage
differential signaling (LVDS) output port.
The new AD9272 was designed to minimize terminated
noise—that is, problems associated with noise generated
by the ultrasound probe heads, a fundamental issue in making
clearer images. It targets high- and mid-end cart-based
ultrasound devices, which are marketed on the basis of topnotch
image quality.
ACCORDING TO THE SPECS...
In terms of noise specifications, the datasheet lists a typical
input-referred noise of 0.75 nV/√Hz at 5 MHz and a gain
of 21.6 dB. (Other gains,15.6 and 18.1 dB, are programmable
via the device’s SPI bus.) But the raw specs need to be
understood in the context of the design concepts explained
in the datasheet.
The anti-aliasing filter combines a single-pole high-pass
filter and a second-order low-pass filter. The high-pass filter
can be configured for dc coupling, or the filter can be configured
at a ratio of the low-pass filter cutoff. This is selectable
through the SPI.
The 12-bit ADC uses a three-stage pipelined architecture—
a 4-bit first stage followed by eight 1.5-bit stages, and
a 3-bit flash. Each stage provides sufficient overlap to correct
flash errors in the preceding stages. The quantized outputs
from each stage are combined into a 12-bit result in the digital
correction logic.
The pipelined architecture lets the first stage operate on a
new input sample and the remaining stages operate on preceding
samples. Conversion rates from 10 to 80 Msamples/s
are possible. The signal-to-noise ratio (SNR) is specified at
70 dB, with spurious-free dynamic range (SFDR) at 75 dB.
Where the AD9272 is intended for high-resolution plug-in
applications, the AD9293 is for portable apps. It’s optimized
for power efficiency, addressing the needs of those portable
ultrasound systems that will be deployed in ambulances and
rescue vehicles.
For that kind of ultrasound equipment, the AD9273 provides
power dissipation of less than 100 mW per channel,
still at 12 bits, but at no more than 40 Msamples/s. It can run
50, but at higher power consumption. At that, you’re talking about 1.2 nV/√Hz, compared to 0.75. On the other hand,
you’re getting the same SNR and SFDR.
That GUI is the key to using the devices’ SPI to customize
the noise and power performance for various imaging mode,
probe, or power requirements (Fig. 2). By changing SPI registers,
designers can optimize an ultrasound signal processing
architecture for noise performance or battery life.
That, as much as the tweaking of the designs of these
two chips, may be what signals that these devices represent
a true second generation in ultrasound AFEs. It means that
there’s growing market acceptance of the idea of a long-term
partnership between integrated device manufacturers and
original device manufacturers in medical products. DON TUITE
ANALOG DEVICES • www.analog.com
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