Medical ultrasound requires the control and processing of a variety of high-speed signals. Those signals include high-frequency sound waves, high-frequency/wide-dynamic-range continuous/pulsed waves, high-speed digital processing and video displays. The challenge facing many circuit designers is to combine all those high frequency signals while facing severe constraints on power consumption, circuit-board area and cost.
Ultrasound research, development, and commercialization have sprouted in the last four decades. It wasnt until the late 60s that the first commercial ultrasound scanner became available for cardiology, neurology and ob/gyn applications. The next major breakthrough came with the introduction of gray-scale imaging, followed by real-time gray scale scanning. Another major advancement was introduction of color Doppler, which is used to determine the velocity and direction of blood flow.
An ultrasound instrument for imaging interior portions of the human body is a sophisticated system; it comprises many high-speed processing elements and subsystems. The underlying concept behind ultrasound imaging is similar to that of sonar. A sound wave is transmitted from a transducer or transducer array, which also "listens" for the reflected signal (Figure 1). By using signal processing techniques to combine the reflected signals, and performing this process over a wide scan area, an image can be constructed to profile the area. Unlike sonar, ultrasound operates at high frequencies (1 to 10 MHz), penetrates to depths of many centimeters inside the human body, and can be used to create 1-, 2-, and 3-dimensional images.
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Analog beam-forming (ABF) ultrasound systems have multiple analog front end (AFE) channels (See Figure 2). The variable-gain amplifier is needed to compensate for attenuation in the medium being penetrated. The time delay element is used to maximize the signal-to-noise ratio of the reflected signal from a predetermined point source (focal zone). Corresponding points on the time-delayed signals from each channel are summed, compressed and amplitude detected (rectified). Analog-to-digital converters (ADCs) process image (8-10 bits, 20 MHz), audio (baseband 12 and more bits at audio sample rates) and color Doppler information (up to 12 bits at up to 10 MHz).
Digital data is processed with the use of FPGA, fixed-function off-the-shelf digital components, and digital signal processors (DSPs). The real-time capability of ultrasound requires optimization by digital processing (which includes FIR, IIR filters and FFT processing). The digitized data--in polar coordinates--must then be processed and mapped into rectangular coordinates, stored in buffer memory, and sent to the video and audio encoders.
Digital beam forming (DBF) systems replace the time delay element per channel with an ADC per channel and storage of successive signal elements in buffer memory (See Figure 3). The converter will typically be clocked at 40 MHz and will require 10-bits of resolution.
HIGH-SPEED IC COMPONENTS FOR ULTRASOUND
Switching: In ABF systems high-speed multiplexers are used to create a cross-point switch. The switch is used to choose a predetermined time delay per channel by connecting each receive channel to a lumped passive LC element or active circuit element. Multiplexers must exhibit low Ron and fast Ton/Toff switching characteristics. Switch settling times >100+ ns are not fast enough for multiple measurement points (gates) during a single scan line. Quad high speed switches like the ADG201HS, ADG411 and ADG441/2/4 offer fast Ton/Toff switching speeds.
 Ton max Toff max Faxcode*
ADG210HS 50ns 50ns 1493
   
ADG411 175ns 145ns 1503
ADG441/2/4 110ns 60ns 1513/4/5

l        For data on these products, call ADIs AnalogFax™ line, 1-800-466-6212, and enter the appropriate Faxback code.