Wireless temperature sensor integrated circuits with on-chip antennas

Abstract

Two wireless temperature sensors with on-chip antenna realized in 0.35 μm CMOS technology are presented. The first sensor exploits a double 3-stage ring oscillator structure, which transform the silicon substrate temperature variation into a frequency modulation. The second device is based on a PTAT (proportional to absolute temperature) sensing element, where the base-emitter voltage difference between two diode connected PNP BJT's with different emitter currents is measured. Temperature information provided by the sensors are transmitted by means of small loop antenna structures which are realized by aluminium deposition on the top surface of the chip, eliminating the need for external connection and sophisticated packaging. The first sensor measurements show a 2.2 GHz oscillation frequency vs. temperature at 3.2 V bias voltage, for each ring oscillator. The double oscillator technique allow us to use a mathematical procedure between two different signals, in order to extract a reliable information on temperature, regardless of the frequency vs. bias voltage variation showed by ring oscillator, with the goal of avoiding a voltage regulators. The second sensor measurements show a wide voltage swing vs. temperature at 3.3 V bias voltage: this results allow us to convert this voltage in a 8-bits digital signal, in order to drive a RF-transmitter using the On-Off Keying (OOK) modulation.