Microelectronics for the thumb-size ultrasound measurement system

Abstract

This thesis presents building blocks and strategies to reach the goal of a thumb-size autonomous ultrasound measurement system with wireless communication capabilities. The design of modern electronics is based on the possibility to accurately predict system behavior by the use of simulation tools. This paradigm can be extended to components such as sensors attached to the electronics. The ability to simulate mechanical components and electronics together renders possible effective optimization at system level, i.e. minimizing size, cost and power consumption. All of these parameters are important for measurement systems aiming at the rapidly growing field of sensor networking and ambient intelligence. The work in this thesis connects the mechanics of piezoelectric transducers with the design of on-chip microelectronics. Throughout the work, SPICE models of the ultrasound system are used within the design tool for integrated circuits. Improvements and verifications of existing SPICE models for ultrasound equipment is described and applied in the design of integrated electronics for an ultrasound measurement system. The overall aim has been to achieve minimal system size and power consumption through interdisciplinary work based on knowledge within both ultrasonics and electronics. The thesis is divided into introductional chapters and eight attached papers. The introductional chapters give an overview of ultrasound devices, measurement technology, and simulation models. Tools and design flow for analog and mixed signal integrated circuits are discussed. Finally, an overview of the electronics in a pulse-echo ultrasound system is given. The attached papers cover various topics required to reach the goals presented above. The first three papers are closely related to the SPICE models of the piezoelectric devices and the ultrasound propagation media. First, a design strategy towards an area optimized driver stage for piezoelectric crystals with the help of SPICE simulations is presented. A prototype chip design has been made and it is shown that simulations and measured performance agree well. Second, diffraction loss for ultrasound pulses is included in the SPICE models for the ultrasound propagation medium. It is shown that this enables accurate simulations of absolute amplitudes in a pulse-echo ultrasound system with its associated electronics. Third, the influence of parasitic components stemming from cabling in the system are simulated and shown to agree well with measured data. Analog to digital converters and comparators are useful components in a pulse echo ultrasound system. The design of these blocks with focus on low power consumption is presented in two papers. The fourth paper presents a low power, high resolution sigma delta A/D converter, while the fifth paper introduces the deign of a delay-time stable time-continuous comparator suitable for use in time quantization A/D converters. The last three papers address three aspects of system level design of the ultrasound measurement system. The sixth paper presents the mounting of the electronics chip directly onto the piezoelectric crystal using wire bond technology. The setup enables precise pulse control and results in a very compact design. The seventh paper discusses the design of a complete embedded internet system (EIS) confined in the space 25x23x5 mm^3. The system incorporates an integrated web server, Bluetooth communication, and TCP/IP stack and is intended to serve as a base for Internet connected sensors. Finally, the eighth paper introduces a complete on-chip solution for the transmission and reception of ultrasound pulses with a piezoelectric crystal. The chip is designed in a high voltage process, and incorporates an inductive boost pump to achieve a high voltage for the excitation of the crystal. An integrated amplifier is used to amplify incoming pulses. The chip is controlled by a digital state machine used to achieve intermittent operation of the amplifier for minimum power consumption.Godkänd; 2004; 20061025 (haneit