Low-power high-efficiency fully duplex wireless interface for implantable electronic devices

With the development of microelectronics and semiconductors in the past decades, there has been a steady growth in production of integrated circuit (IC) design for implantable biomedical systems, which can help physician's diagnosis and restore functions in case of analysis and disability in modern medicine. In these systems, one external controller sends command to implantable unit and processes received data from the implants through an interface unit. One interface unit at implant side needs recovering data and powering from external controller. Among few reported approaches, inductive powering biotelemetry system has been widely accepted as an ideal method for the reason that it requires neither transcutaneous leads nor implanted batteries. However there are still a number of challenges regarding implementation of such interface module. One main challenge comes from the inefficient power transferring of the present inductive links due to inherent low coupling-coefficient of the weak inductive coupling and power losses of power recovery chain. Another challenge comes from data communication specification of some Functional Electrical Stimulation (FES) systems that require high forward transmission data rate. And backward data transmission for monitoring and controlling is desired in most systems. However, the nature of narrow-band inductive link poses a great challenge to design of bi-directional data communication with such high data rate. The third challenge stems from miniaturization requirement of the implantable system since restricted space of the system raises the effective cost of every additional off-chip component.; In this project, we have achieved two main contributions with respect to the development of the wireless interface module. First, an efficient integrated wireless interface module is proposed and implemented. We analyzed power transferring efficiency of the overall power recovery chain and presented the way of implementing a regulation loop to reduce the power losses of overall system. This power regulation loop, which is able to optimize the transferred power regardless of coils movement and load variation, also reduces potential hazards of patients exposed to high electromagnetic fields. As parts of the power recovery module, two linear low dropout regulators and switched-capacitor DC/DC converters using state-of-art CMOS technology are presented. Power-on-detect circuit techniques are proposed to improve the power supply rejection ratio (PSRR) of the system. We also investigated several different modulation methods for biotelemetry applications from viewpoints of the effects on power transfer, maximum data rate and their own power consumption. Furthermore, we proposed using phase-shift-keying (PSK) as the forward data transmission modulation method and developed a fully integrated low power binary PSK demodulator based on COSTAS loop. Measurement and simulation results prove that the technique in conjunction with load shift keying (LSK) technique can realize full-duplex bi-directional data communication in a narrow-band inductive coupling system, which improves system's ability of control and diagnosis. Second, as power efficiency of the whole system is of our primary concerns in this project, state-of-the-art low power analog circuit techniques have been investigated. A low power 900mV sigma-delta modulator is developed and implemented as one critical block in proposed power control loop. Its characteristics also satisfy most requirements of neural signal recording applications. Some other low-voltage low-power analog circuits dedicated to implantable electronic devices, for instance a current mode bandgap reference and a rail-to-rail Opamp, are also implemented and presented.