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ANALOG & MIXED SIGNAL DESIGN solutions. Thus, it is now common to find octal receivers including the LNA, VGA, AAF, and ADC in packages as small as 10x10mm. High-voltage pulsers are also now available in 4- and 8-channel single-package configurations as small as 10x10mm. These advances are significant and have played a key role in enabling the current generation of portable Fig. 3: Transmit receive (TR) switch has nine discrete components. In a 128-channel system, there are over 1000 discrete parts in these switches alone. systems. Moving forward, however, there are more integration opportunities. The MAX2082 octal transceiver – see figure 2 - is an example of the latest advance in highly integrated ultrasound solutions. It includes the full receiver, TR switch, coupling capacitors, and the three level high-voltage pulsers in a single 10x23mm package. This single transceiver saves considerable space, reduces design time, and lowers overall system cost. The space savings from such a highly integrated transceiver can be dramatic. The integrated TR switch alone represents significant savings. Consider a typical discrete TR switch used in most existing ultrasound systems – see figure 3. There are nine discrete components in this TR switch implementation. In a 128-channel system this represents over 1000 discrete parts for the TR switch function alone! Figure 4 shows a PCB layout using the MAX2082 for a 128-transceiver channel configuration. The space required is less than 10 square inches, which is less than half the space required for current solutions that use individual octal receiver ICs, octal pulser ICs, and discrete TR switches. Fig. 4: 128-channel PCB layout using octal transceivers. Transceiver power management Power is also a major concern in these highly integrated designs. Many of these ultrasound systems are portable and must run from a battery for an hour or more between charges. Heat management is also problematic as the component density is very high and the PCBs can be very close together, leaving little room for airflow. The ultrasound transceivers represent a significant portion of the overall system power budget and, therefore, warrant significant design attention. Over the past 10 years ultrasound receiver power has been cut in half. It is now common to find IC receive solutions including the LNA, VGA, AAF, and ADC that burn less than 150mW per channel. These new-generation receivers also have more flexible power-control features allowing users to trade off power for performance as well as utilize low-power, fast wake-up “nap” modes to conserve power when systems are in non-imaging modes. There are still more opportunities for future improvement. The TR switch itself, for example, can burn in excess of 80mW per channel, because significant bias current is required to lower the on-impedance of the diodes to meet necessary noise performance. This is almost as much power as the rest of the receiver! Newer proprietary integrated TR switch designs in products like the MAX2082 transceiver mentioned above achieve better noise performance than these discrete designs for less than 15mW per channel. Balancing noise with miniaturization High levels of integration and low power are obvious design challenges for portable ultrasound systems. Not so obvious are some of the performance issues associated with the miniaturization of this equipment. Minimizing in-band noise Ultrasound systems are extremely sensitive to both radiated and conducted in-band noise and interference in the 2MHz to 15MHz range. The input sensitivity of a single channel can be as low as 1nV/rtHz. In typical 128-channel systems, an unwanted signal applied to all inputs can have a processing gain of up to 21dB, depending on channel-to-channel beamforming delay. As a result, an in-band noise signal applied to all inputs as small as 0.09nv/rtHz can be visible and appear as an artifact in the image. These artifacts occur so commonly that they are universally called “flash light” artifacts; they resemble a beam of light in the center of a phased array image where the system has the highest processing gain to a common input signal. Signals this small can easily come from a variety of radiated or conducted interference sources in the system. Ultrasound system designers go to great lengths to physically separate and shield noisy digital circuitry from sensitive analog circuitry, and to control ground loops. Unfortunately, portable ultrasound system designers do not have the luxury of physically separating this circuitry, and shielding can be problematic given the limited space and heat density of most PCBs. As a result, it is extremely common that in-band noise problems occur in these designs, especially when they are so physically close to noisy single-board PCs commonly used to perform many of the computational and display tasks. Consequently, it is particularly important that proper attention to grounding and shielding be made early in the design process. Trying to modify these highly integrated designs later in the prototype evaluation phase can be extremely difficult and time-consuming. Minimizing audio noise In many cases, low-frequency audio noise can also be a problem and, in fact, is often more difficult to solve. In ultrasound systems blood flow is detected by measuring the small Doppler frequency shift of the reflected transmit signal. Any low-frequency modulation of the transmit signal or the received signal from stationary objects will produce noise sidebands which Fig. 5: Doppler near-carrier noise. 22 Electronic Engineering Times Europe January 2014 www.electronics-eetimes.com


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