Wirelessly Powered Sensor Transponder for UHF RFID

A transponder for UHF RFID with sensor interface for external capacitive sensors is described. The sensor interface consists of a capacitive voltage divider and a 10 bit successive approximation analog to digital converter. The power consumption is about 30 μW
_DC
and the transponder is wirelessly powered with RF waves from the reader device. Wireless measurement of capacitance is demonstrated at a distance of 30 cm with 0.5 W erp transmission power. Low operation distance is mostly due to low efficiency of the rectifying Schottky-diodes in the particular BiCMOS process.