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This paper presents a prototype RFID humidity sensor capable of passive wireless sensing through far-field backscatter coupling. A commercial UHF RFID tag is employed as a sensing platform to receive the power and to reflect the sensed data back to the RFID reader. A humidity sensitive polyimide film is incorporated onto the top surface of the RFID tag for humidity sensing. The prototype sensor demonstrated that power required to activate the sensor tag is a linear function of relative humidity and the maximum sensing distance between the reader antenna and the RFID humidity sensor tag reaches to about 1.5 m. Due to its unique features of low cost, battery-less wireless operation, maintenances-free, and disposable, the RFID humidity sensor can be integrated into wallpaper for humidity monitoring in a built environment.

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An ultra-low-power four-channel sensor interface for wireless sensor networks employing semi-active RFID transponders as sensor nodes is described in the present paper. Each channel is based on a self-contained switched-capacitor second-order single-bit Sigma Delta-modulator operated in an incremental manner. The power consumption can be regulated through variation of the sampling frequency (max. 1MHz), signal bandwidth (8Hz-16kHz), output resolution (max. 16 bits) and/or output word rate. The input and output stages of the employed amplifiers are both class AB for a high energy efficiency. Moreover, the bias current of every amplifier can be programmed according to the actual power availability, thus dynamically exchanging DC-gain, gain-bandwidth-product and slew-rate against power consumption. In addition, two channels may be cascaded (2-2 MASH configuration), further incrementing the resolution and/or reducing the number of clock cycles to obtain a given resolution. The MASH configuration uses an on-line adaptive noise cancellation scheme to further enhance the noise-shaping characteristic of the converter. The power consumption per channel ranges from 4 to 20 mu W at a supply voltage of 1.4