A 0.9-V Analog-to-Digital Acquisition Channel for an IoT Water Management Sensor Node

Abstract

This brief presents an analog front-end (AFE) specially developed for a water conductivity measurement system for water network management applications. Water conductivity is one of the best ways to detect a problem with the water in the network. In order to trigger an alarm it is only required to detect a 10% variation, which allows to simplify the system design. The proposed sensor uses four platinum micro-machined electrodes, where a 6 kHz signal is applied to the outer electrodes and a voltage signal is read in the inner ones. A programmable switched-capacitor (SC) bandpass filter is included in the analog-to-digital (A/D) channel to attenuate the interference from the 50 Hz power network, that shares the same environment with the water network, and to allow using a square wave signal to drive the sensor, thus simplifying the overall system. The resulting sine wave is digitized using a passive continuous-time (CT) 2nd order $\Delta\Sigma$ modulator. The output bitstream is decimated, using a sinc filter, and the RMS value of the voltage is calculated. The A/D acquisition channel was implemented in a 130 nm CMOS technology, using a supply voltage of 0.9 V and a clock frequency of 1 MHz (SC amplifier and SC bandpass filter) and 4 MHz ( $\Delta \Sigma $ modulator). Measurement results show a SNDR of 44.4 dB for an input signal of 6 kHz and an amplitude of −41 dBV. The channel dissipates 386 $\mu \text{W}$ and occupies an area of 0.27 mm2.