RADIO-EXTINCTIVE SOIL ACIDITY TRANSDUCER BASED ON ISFET-TRANSISTOR

Abstract

This work addresses the scientific and practical challenge of improving the accuracy and energy efficiency of soil acidity measurements directly in field conditions. Based on theoretical analysis and SPICE modeling, a measuring converter circuit was developed using an ISFET transistor with a Ta₂O₅ sensing layer, providing an input impedance of up to 1 GΩ. The implementation of the OPA333 precision amplifier with Zero-Drift technology minimized systemic errors and eliminated the need for complex high-voltage electrometric circuits. A key feature of the design is the hardware-based temperature compensation of the Nernst drift: integrating a Pt1000 thermal resistor into the feedback loop ensured automatic gain adjustment across a temperature range of 0–50°C. This solution significantly reduces the computational load on the microcontroller and allows for the most efficient use of the ADC dynamic range. To ensure reliable operation in environments with industrial noise, a passive RC filter (τ = 1s) was applied, guaranteeing noise suppression at a level of -50 dB. Although this results in a signal settling time of approximately 5s, such inertia is optimal for stabilizing the signal when monitoring slow processes in the soil environment. Metrological studies confirmed high linearity of the conversion characteristic (R² > 0.99) across the pH 0–12 range, while the calculated resolution of 0.004 pH fully meets the requirements of modern agrotechnical monitoring. Energy consumption analysis confirmed the high autonomy of the device: the power consumption of the analog tract is only 0.62 mW. This constitutes less than 1% of the energy budget of a typical LoRaWAN node, allowing the interface to be integrated into wireless stations powered by 18650-type batteries without frequent maintenance. The practical significance of the results lies in the possibility of creating reliable and cost-effective tools for precision farming systems.