A passive UHF RFID system with a low-power capacitive sensor interface

In this work, a passive UHF RFID system for capacitive sensor applications is presented. The design of the base station and the passive RFID tag is explained. The tag consists of an inductively-fed, meandered dipole antenna and an IC that contains the remote powering, communication, and sensor interface circuitry. The rectifier in the tag is a highly efficient differential rectifier with self threshold cancellation. A low-power, all-digital, PLL-based sensor interface is used to read the differential capacitive sensor on the tag. The pulsewidth modulated sensor data is sent to the base station by backscattering. The base station is implemented using discrete components with a custom designed patch antenna. The reflection from the tag antenna under different loading conditions is measured in the anechoic chamber. The whole system is measured and it is shown that the sensor value can be read from the base station. The tag chip is implemented in 0.18 μm UMC CMOS process.