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

This paper 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 non-idealities of the circuit components. The design consideration is provided for different components. A prototype PCB of the proposed circuit is developed. The results show that the proposed circuit can provide a signal-to-noise ratio of 80 dB for a wide-range incremental change in the sensor resistance up to 500 kΩ 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.