Low-Cost Capacitive Sensor Front-End Circuit Based on Phase-Locked Technique

Abstract

This research presents an efficient front-end circuit for grounded capacitive sensors (GCS), based on a phase-locked technique. An all-pass filter serves as a phase shifter to obtain the desired 90-degree phase difference, enabling the sensor capacitance to be calculated from the locked frequency regardless of parasitic capacitances. For long-distance capacitance measurements (remote sensors), the proposed circuit integrates an improved active shield technique, allowing real-time measurements using only a low-cost microcontroller unit (MCU), display, and commercially available electronic components, without the need for expensive or bulky instruments. The design is fully automated and does not require manual adjustments. Simulation and experimental results are consistent and validate the effectiveness of the proposed method. Comparative experiments with standard instruments demonstrate accurate measurement within the 1–500 pF range through shielded cables up to 10 meters, while maintaining a maximum nonlinear error (NLE) of only 0.04% full-scale span (FSS). The proposed circuit architecture effectively mitigates the effects of parasitic cable capacitance and the input capacitance of operational amplifiers. This approach provides a low-cost, compact, fast, and highly accurate solution, making it suitable for applications of liquid level measurement in hazardous or hard-to-access environments through remote capacitive sensor measurement.