Continuous-wave (CW) Doppler ultrasound diagnostic systems involve many pairs of transducers. One member of each pair transmits a continuous sinewave. The other receives echoes. A CW ultrasound diagnostic device works by performing spectral analysis on the signals from the receive transducers. In addition to medical imaging, applications that require similar measurements of phase shifts include weather radar, adaptive antenna arrays, and automotive collision avoidance systems.
In a blood vessel, the sound waves have their highest velocity in the center, with decreasing velocity closer to the blood vessel wall. An obstruction in the vessel will alter the flow pattern.
Typical arterial blood flow has forward, reverse, and second flow components. When there's an obstruction, some or all of the second or reverse flow components are absent. Often, the person administering a CW Doppler test will ask the patient to cough. A lack of change in the flow indicates a blockage in the artery.
A monolithic dual-channel I/Q demodulator and phase-shifter IC from Analog Devices, the AD8333 makes it easier to develop CW Doppler systems. The phase-shifter features a 161-dB/Hz dynamic range, which is a critical specification in the application. The AD8333's dynamic range is as good as, or better than, larger-footprint discrete-component designs. On top of that, it uses 190 mW/channel.
CW Doppler designs typically require beamforming in the analog domain. However, this implies a large dynamic range that can't easily be processed digitally. Consequently, system designers have had to rely on analog beamforming (ABF) by using multiple low-noise amplifiers, analog delay lines, and numerous cross-point switch matrices.
The AD8333 replaces this approach with a programmable phase-shifting technique. Here each input channel has an I/Q demodulator, eliminating the need for analog delay lines and crosspoint switches.
The AD8333 uses 22.58 phase steps. Parallel 4-bit digital control of phase states is inherent, as is accurate channel matching through a common local-oscillator interface. Complementary ADI chips include dual and quad variable gain amplifiers, a clock IC, high-speed amplifiers and line drivers, and a selection of analog-to-digital converters.
Currently sampling, the AD8333 will be in full production in August. Pricing for the amp, packaged in a 5- by 5-mm LFCSP, runs $8.95 each in 1000-unit quantities.
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