A Compact Multi-Channel Neural Stimulator with a High-Efficiency Wireless Power and Data Transfer System for Batteryless Invasive BCIs

This paper proposes a compact multi-channel stimulator with a high-efficiency wireless power and data transfer system for invasive brain computer interfaces (BCIs). The extracorporeal part transmits power and data to the intracorporeal part through a single pair of coils. An adaptive active rectifier operating at a frequency of 10 MHz achieves zero voltage switching under a hybrid delay time controller. Frequency shift keying (FSK) is employed for the downlink data transmission. The active rectifier achieves maximum output power greater than 200 mW and peak power conversion efficiency (PCE) of 94%. To realize constant current stimulation, a two-stage programmable charge pump is designed for high voltage generation. The neural stimulator can be flexibly configured to generate a stimulation current of $0.1 \sim 12 \mathrm{~mA}$ through an 8-bit IDAC, realizing a bi-directional rectangular waveform of 1 333 Hz with adjustable pulse width. The system includes an impedance measurement circuit that monitors electrode-tissue impedance using a 10-bit SAR ADC. The proposed stimulator is fabricated in a $0.18-\mu \mathrm{m}$ BCD process, and occupies a total area of $4.62 \mathrm{~mm}^{2}$.