Current-Excitation-Based ΔR-to-Frequency Converter for Resistive Sensors

This article presents a relaxation oscillator-based high-resolution resistance measurement system. The proposed signal conditioning circuit utilizes a half-bridge topology with a bidirectional current excitation. The circuit modulates the incremental change in the sensor resistance in the form of a change in the frequency of the relaxation oscillator. The baseline resistance of the sensor is automatically compensated using a digital potentiometer at the beginning of the measurement cycle. The proposed circuit is analyzed in detail by considering the nonidealities of the circuit components. The design consideration is provided for different components. A prototype printed circuit board (PCB) of the proposed circuit is developed. The results show that the proposed circuit can provide a signal-to-noise ratio (SNR) of 80 dB for a wide-range incremental change in the sensor resistance up to 500 k $\Omega $ with an error of less than ±1%. The developed board is tested with humidity sensors. The result confirms the efficacy of the proposed circuit for a wide measurement range.