A time domain bridge-based impedance measurement technique for wide-range lossy capacitive sensors

• We proposed two interface circuits for impedance measurement of lossy capacitive sensors.
• Separate measurement of capacitance and lossy resistance is possible through this approach.
• We investigated the impedance measurement range for both the configurations.
• Range and precision is improved by bridge auto-balancing technique through quadrature component compensation.
• One of the proposed configuration is tested with a fabricated lossy capacitive humidity sensor and the verified results are reported.

In this paper, a new, simple and accurate impedance measurement technique (IMT) for wide range lossy capacitive sensor is proposed. The proposed technique accepts sensors having capacitance ‘Cx’ with lossy resistance ‘Rx’ in parallel. The proposed IMT is based on the separation of in-phase and quadrature components, which is achieved by using phase to time converter. For comparative study, two circuit configurations are proposed. In first proposed solution, the sensor parameters are measured by separating the real and imaginary part of the output electrical signal. In the second proposed configuration, the measurement range is enhanced using an auto-balancing approach, which is achieved by monitoring phase difference between input and output voltages. Experimental evaluation of the proposed configurations reveals that the first configuration is able to estimate the sensor capacitance in the range 200–1200 pF and lossy resistance in the range of 100 kΩ–1.2 MΩ with a maximum error of 6%. The second configuration exhibits an enhanced range of 100–2000 pF for sensor capacitance and 33 kΩ–3 MΩ for lossy resistance with a maximum error of 5%. The IMT was interfaced with a fabricated γ-Al2O3 based impedance humidity sensor, measured its Cx (70 pF–800 pF) and Rx (35 kΩ–4 MΩ) at different concentration of humidity, calculated the humidity, and compared its performance with existing techniques, showing the practicality of the proposed IMT.