An Adaptive Impedance Matching System with Absolute Peak Detection for RF Energy Harvesting

Distributed neural implants with multiple wireless microsystems face the challenge of varying orientations and distances from an external RF power transmitter, leading to spatial variations in coupling coefficients and different power levels received by each device. This variation affects the rectifier’s input impedance, making a fixed impedance matching network (IMN) inefficient. This paper presents an autonomous, low-power method using a switched capacitor array to dynamically tune the impedance matching network of RF energy harvesters under varying coupling conditions. The proposed absolute peak detection circuit operates within a voltage range of 0.24 V to 1.4 V and has a peak error of 0.19 mV at 1.2 V, autonomously identifying the optimal impedance matching condition to maximize the harvested power. The system demonstrates improvements in rectifier output power by 11% and 45% for coupling ratios of 0.015 and 0.035, respectively, while consuming only $1.24 \mu \mathrm{~W}$.