A Monolithically Packaged Cordless Sensor System Embedding MEMS A/D Converter and Saw Transponder

A monolithically packaged SAW (Surface Acoustic Wave) radio transponder and pressure sensor is developed for the application to a TPMS (Tire Pressure Monitoring System). The device contains the wireless transponder, which converts analog signal into digital one without any auxillary electronic circuits and then transmits the converted data wirelessly. No power sources are needed for wireless transponder and pressure sensor. The touch-mode pressure sensor converts externally applied pressure into a capacitance, and the SAW radio transponder radiates sensor values as pulse train to the interrogation (measurement) unit. The realization of the mechanical A/D conversion is possible since the SAW radio transponder has been connected to the touch-mode capacitive pressure sensor. The SAW radio transponder and touch-mode sensor have been fabricated using a surface micromachining and a bulk micromachining technologies, respectively. The performance of the integrated, passive and wireless pressure sensor meets the design specifications such as linearity, sensitivity and noise figure. Finally, experimental results on the radio transponder and sensor without power source are presented.