ANALOG FRONT-END CIRCUITS FOR HIGHLY INTEGRATED ULTRASOUND IMAGING SYSTEMS

Ultrasound imaging is a well-established medical diagnostic technique. Compared with other imaging modalities, such as for example X-ray, ultrasound is harmless to the patient and less expensive while providing real-time imaging capability with adequate resolution for most applications. Piezoelectric materials have dominated the ultrasound transducers technology for a long time but, thanks to the intense research activity in recent years, capacitive micromachined ultrasonic transducers (CMUT) are emerging as a competitive alternative for next generation imaging systems. The objective of the thesis is to analyze the ultrasound system, when a CMUT is used instead of a piezoelectric transducer, to identify and design the best integrated solution to optimize the front-end performance. After giving an overview of the ultrasound system and the Capacitive Micromachined Ultrasonic Transducer (CMUT) in Chapter 1, Chapter 2 presents a thorough comparison between RX amplifier alternatives. The impact on the pulse-echo frequency response and SNR is assessed. The study demonstrates that a capacitive-feedback stage provides a remarkable improvement in the noise-power performance compared to the very popular resistive-feedback amplifier, at the expense of a low-frequency shift of the pulse-echo response, making it suitable for integration of dense CMUT arrays for low and mid-frequency ultrasound imaging applications. Then, Chapter 3 proposes the design of a CMUT front-end circuits comprising a TX driver, T/R switch and RX amplifier. Realized in BCD8-SOI technology from STMicroelectronics, the TX delivers up to 100V pulses, while the RX shows 70dB dynamic range with very low noise at 1mW only power dissipation. Measurement results and imaging experiments are presented and discussed. In Chapter 4, the non-linear behavior of the CMUT is discussed and possible solution proposed. Experimental results demonstrate a significant reduction of the second-harmonic distortion, estimated to be lower than -30 dB, resulting in good linearization for typical nonlinear imaging operation. In addition, Chapter 5 shows a novel amplifier architecture exploiting the regeneration feature of the cross-coupled pair. It will be used as Programmable Gain Amplifier (PGA) in the ultrasound chain. A test-chip in 0.18 μm CMOS provides 15dB to 66dB gain over 50MHz bandwidth. With state-of-the-art noise and linearity performance, a record GBW up to 100GHz is demonstrated with only 420 μW power dissipation.