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ANALOG & MIXED SIGNAL DESIGN Designing compact medical ultrasound systems By John Scampini Over the past decade, medical ultrasound imaging equipment has experienced a revolution. Advances in integrated electronics have allowed equipment manufacturers to significantly improve the portability and the affordability of this powerful medical tool. What once weighed hundreds of pounds and required a cart for mobility, is now the size of a portable laptop computer. Needless to say, the impact for the medical community and the patient has been profound. In developed countries ultrasound is now utilized at the patient’s point of care, which reduces costs and improves outcomes. In developing countries ultrasound imaging is now available to a much larger segment of their large rural populations. The impact of these new, more portable low-cost systems on healthcare worldwide is significant and well received. The future of the technology promises even more advances. The road to developing these compact imaging solutions has not been easy. There have been, and will continue to be, significant design challenges Fig. 1: Ultrasound transceiver block diagram shows the diverse functionality required. as manufacturers strive to make these systems more portable, less expensive, and higher in performance. This article will highlight some of the more significant Fig. 2: MAX2082 ultrasound transceiver integrates the full receiver, TR switch, coupling capacitors, and the three level high-voltage pulsers. design challenges faced by designers of this equipment. Designers of compact ultrasound systems must fit the large number of ultrasound transceivers required to produce a highquality image into the available small space. This is no simple task. Current state-of-the-art systems commonly possess 128 or more of these transceivers. A typical ultrasound transceiver block diagram is shown in figure 1. To produce an ultrasound image, the transceiver’s high-voltage transmitters generate properly timed high-voltage pulses to excite the ultrasound transducer elements and generate a focused acoustic transmission. Acoustic energy from this transmission is reflected by impedance discontinuities in the patient’s body, received by the same elements, and routed back to the receive portion of the transceiver. The receiver consists of a transmit/receive switch (TR switch), low-noise amplifier (LNA), variable gain amplifier (VGA), anti-alias filter (AAF), and analog-to-digital converted (ADC). Each transducer element is connected to an LNA through a TR switch which protects the LNA input from the high-voltage transmit signal. The LNA itself provides an initial fixed gain to optimize the receiver’s noise performance. The VGA is used to compensate for the attenuation of the ultrasound signals in the body over time which reduces the dynamic range requirements for the subsequent ADC. The AAF in the receive chain keeps any high-frequency noise beyond the normal maximum imaging frequencies from being mapped into the receive band by the ADC. The amplified and digitized signals are delayed and summed in the ultrasound system’s digital beamformer to generate a focused receive beamformed signal. The resulting digital signal is used to generate 2D images as well as pulsed-mode Doppler information. The receiver also has a separate continuous wave Doppler (CWD) receiver/beamformer path after the LNA. In CWD mode, the receiver’s dynamic range requirements are very demanding and beyond the range of the VGA/ADC signal path. CWD beamforming can be achieved by mixing the received signals with properly phased LOs and summing the resulting baseband signals. As a result, the CWD receiver block is comprised of high-dynamic-range analog I/Q mixers and programmable LO generators. As one can see, there is significant functionality in a typical transceiver and getting 128 or more of these into something the size of a PC is a design challenge. Analog IC manufacturers have responded to this challenge with more highly integrated John Scampini is an Executive Director at Maxim Integrated in the Industrial Communications and Ultrasound Business Unit - www.maximintegrated.com www.electronics-eetimes.com Electronic Engineering Times Europe January 2014 21


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