An ultra-low-power sensor interface built around a reconfigurable incremental sigma-delta modulator for sensor networks employing electromagnetic backscatter

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 SigmaDelta-modulator operated in an incremental manner. The power consumption can be regulated through variation of the sampling frequency (max. 1 MHz), signal bandwidth (8 Hz-16 kHz), 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 muW at a supply voltage of 1.4 V.