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 0.18μm 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 μW, a throughput power range from 150 μW 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.