Self-Sustaining Soil Electrical Conductance Measurement using a DC-DC Power Converter

This paper presents a self-sustained soil electrical conductivity (soil-EC) sensing IC. The sensing system harvests energy and measures the conductivity of the soil simultaneously using the proposed wide-range DC–DC converter. The power management circuits must accommodate the wide variations of the soil conductivity at varying power levels. The time-domain current sensing mechanism, which is embedded in the DC–DC converter, is employed to measure conductivity. Also, the digital codes are recorded and control the converter for achieving maximal power extraction and maintaining high conversion efficiency. The self-sustained soil-EC monitoring IC was fabricated using 0.18- $\mu \text{m}$ CMOS technology. The range of the input voltage from a soil cell is from 0.8 to 1 V when carbon and zinc electrodes are used. The design achieves a tracking efficiency of 98% over an input range of 416– $5000~\mu \text{W}$ , a throughput power range from $150~\mu \text{W}$ to 4.32 mW, and a peak system conversion efficiency of 88%, while the regulated voltage is about 1.7–1.8 V. The design can also measure the soil EC from 0.114 to 0.744 mS/cm in a resolution of 0.00568 mS/cm per bit.