Meeting converter needs in portables
01 October 2008
Since the development of early ultrasound systems (i.e. cart-based ‘dragable’ ones) the medical industry has used this real-time technology for early detection of health issues as well as general diagnostics procedures
Ultrasound systems have become more portable, with some evolving into ultra compact palm-sized devices. In the not so distant future, an ultrasound system could become a specialised PDA as common as the doctor’s stethoscope.
Until recently, most systems were developed on the component level using discrete solutions and multiple ICs. The receive signal chain consists primarily of an LNA (low-noise amplifier), a VGA (variablegain amplifier), an AAF (anti-aliasing filter) and an ADC (analogue-to-digital converter). Each of these components is replicated many times in common DBF (digital beamforming) architectures. Increasing the number of channels improves the dynamic range as long as the channel noise is uncorrelated. The most
common number of channels for higher-end systems is 64 to 256 channels, with 16 to 64 channels being more common for portable, mid- to low-end ultrasound systems.
In a DBF system multiple channels are spatially summed to develop an image. DBF architectures are
preferred over earlier analogue beam forming systems because they tend to have better channel-to-channel
matching characteristics. Once the signal is acquired, it is digitised, allowing beam steering and coherent
signal summation to be performed to enhance the signal quality. Bringing the digital engine closer to the
ultrasonic sensors enables significantly finer adjustments to be made that could be achieved in the analogue system. Even though DBF is the most commonly-used architecture today, there are some significant challenges, including power consumption due to the large number of channels, and size, due to
the sheer number of components required to acquire and produce an accurate signal.
Why portable?
A large number of demanding portable applications could realise the benefits of a lightweight compact device that delivered real-time scanning. Field EMS teams will have quicker access to a patient, and will be able to send in result ahead of time. If the ride is long, the doctor can diagnose remotely while awaiting the patient at the hospital. General practitioners can quickly scan the patient as part of a general procedure during routine office visits without requiring a specialist.
Increased portability presents more opportunities to bring these devices into remote areas and villages that need medical attention, but do not have the luxury of electrical power.
Veterinary surgeons use portable ultrasound for on-site diagnosis of larger animals and pets. It is also being used for swine and cattle reproduction for those ranches that specialise in food production farming.
Non-destructive ultrasound is also a growing market. More systems are being deployed to scan bridge beams, industrial equipment bearings, and oil pipelines, for example. This cuts down on inspection costs
and the need to bring down expensive equipment at critical times. It also develops more effective preventative maintenance programmes. Portability in this area is key to catching potentially catastrophic problems before they happen.
Adoption of portable ultrasound carries a cost however, not only in acquiring the new devices to diagnose, scan and analyse, but also in training.
Integrated solutions
Silicon vendors are designing integrated components that provide the necessary signal chain blocks together, allowing users to drastically reduce the power and board space required. For example, the
AD9273 from Analog Devices reduces the total area per channel by more than a third in comparison to a discrete solution and consumes only 100mW per channel at 40Msample/sec. Integrated devices like the AD9273 can also offers customising options through an interface such as a serial port, allowing further optimisation of power and configurability, depending on the application.
The AD9273 embodies an eight-channel signal chain, each channel comprising a LNA, VGA, AAF and ADC. This is the receive chain commonly used to process return pulses in pulsed-wave mode: Bmode scanning for grey scale imaging, and F mode, which is a colour overlay on the B-mode display, to show blood flow. In pulsed-wave mode, the transducer alternates between transmission and reception to develop a periodically updated two-dimensional image.
Another common form of imaging is CW (continuous-wave) Doppler, or D-mode, for showing blood flow velocities and their frequencies. The image is produced using continuously generated signals, where one-half the transducer channels are transmitting and the other half are receiving. It has the advantage of measuring high velocities of blood flow accurately, but it lacks the depth and penetration found in traditional pulsed-wave systems.
Since each has its own benefits and limitations, modern ultrasound systems commonly use both modalities. The AD9273 is applicable to both. It allows the user to operate in continuous-wave Doppler mode by employing an integrated crosspoint switch.
This crosspoint switch allows channels of similar phase to be coherently summed into groups for phase alignment and summation. The AD9273 supports delay lines for low-end systems, and the AD8339 quad demodulator with programmable phase adjustment for the best performance. The AD8339 allows finer adjustments to phase alignment and summation in order to increase image accuracy. This device easily connects externally, allowing the user to compact more of the signal chain required for signals that need very
large dynamic range.
Range and noise
As the signals penetrate through the body they are attenuated by about 1dB/cm/MHz. For example, with an 8MHz probe and 4cm depth penetration, the signal amplitude variation from reflections near the surface
will be 64dB (or 4*2*8). Adding 50dB of imaging resolution, and accounting for loss from bone, cables, and other mismatches, the desired dynamic range approaches 119dB.
To put this into perspective, a 0.55Vpp fullscale signal with a 1.42-nV/ˆ Hz noise floor in a 12MHz bandwidth implies a 92dB input dynamic range. Additional dynamic range is achieved by using multiple channels
[10×log(N channels)], e.g., 128 channels increases the dynamic range by 21dB. This establishes a practical limit for dynamic range between 100dB and 120dB.
The achievable dynamic range is limited by the front-end components. Since the entire dynamic range is not needed instantaneously, an ADC with less dynamic range can be used by sweeping the gain of the VGA to match the attenuation of the received reflection over time, or TGC, timegain compensation.
The LNA sets up the equivalent dynamic range that can be mapped into the ADC. An equivalent dynamic range of 92dB in a 12MHz bandwidth (162.7 dB/rt-Hz) is required to handle both very small and large signals (echoes) from the tissue being scanned (see figure 2). The full-scale of the LNA should be large enough not to saturate from the near-field signal and the lower the noise floor the higher the dynamic range.
The maximum gain required is determined by
(ADC Noise Floor/VGA Input Noise Floor) + Margin =
20 log(224/5.5) + 12dB = 44dB
The minimum gain required is determined by
(ADC Input FS/VGA Input FS) + Margin =
20 log(2/0.55) – 10dB = 3dB
Since power increases as the noise level is reduced, some compromise must be made in portable applications because of power constraints. The specific implementation and choice of components are proprietary to each ultrasound manufacturer.
COREY PETERSEN is a senior design engineer, San Diego Design Center; ROB REEDER is a senior converter applications engineer, both Analog Devices
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