An RFID to I2C Bridge IC with Supply Interference Reduction for Flexible RFID Sensor Applications

Abstract

This paper presents a fully passive 13.56MHz RFID to I2C bridge IC, which has been optimized to reduce supply interference, for RFID sensor applications. To reduce the supply interference, this paper proposes a modified low-dropout regulator (LDO) topology. Firstly, the LDO utilizes NMOS as pass transistor to reduce the interference in the main power path. In addition, a charge pump is implemented to increase the supply voltage of the error amplifier, so that the error amplifier is isolated from the supply interference as well. Finally, a bandgap voltage utilizes a low-pass filter so that it carries also minimum interference. In order to support enormous high-precision I2C sensors and eliminate the micro-controller in the application, an I2C master is integrated on-chip directly. This minimizes the complexity of the overall system. This design utilizes an internal volatile memory to exchange the communication information between RFID and I2C. Therefore the RFID and I2C communication operate independently, so that both communications can maintain maximum compatibility and flexibility. The chip is fabricated in a standard 0.35μm CMOS technology and mounted to a tuned RFID transponder coil on an FR4 PCB substrate. Measurements show that the ripple reduction of this proposed LDO reaches 26 dB from 400 Hz to 25MHz in the worst case. During the communication, the ripple of the supply voltage V DDA reaches a maximum 44 mV, while the input ripple of the unregulated DC voltage V DCA reaches maximum 892 mV. The RFID to I2C bridge was employed in a demonstrator for wireless real-time measurement of temperature, humidity, and pressure using two commercially available sensor ICs and two RFID readers.