UBC Theses and Dissertations
Capacitance readout circuits based on weakly-coupled resonators Hafizi-Moori, Siamak
Capacitive sensors and their associated readout circuits are well known and have been used in many measurement applications in different industries. Improving the sensitivity, resolution and accuracy of measuring small capacitance changes has always been one of the important research topics, especially in recent years that sensors are becoming smaller in size with lower associated capacitance values. This thesis focuses proposes a new method for implementing capacitance readout circuits with higher sensitivity. This is the first time, to our knowledge, that this method has ever been applied directly in electrical domain for capacitance measurement applications. The proposed method, which is based on weakly-coupled-resonators (WCRs) concept, can achieve considerably (orders of magnitudes) higher sensitivity while simplifying the analog front end circuitry and reducing the cost. For comparison, capacitance-to-frequency conversion readout circuits were chosen, which are one of the most reliable and best performing designs and also the closest to our WCR method since both involve shift in natural modes due to capacitance changes. Analysis and SPICE simulations followed by experiments proved the concept. The experimental results have shown almost two orders of magnitude higher relative sensitivity for our two-degree-of-freedom (2DOF) WCR-based system. In the next step we proposed a novel (named hybrid) method to reduce the measurement error considerably (4 to 6 times lower). Hybrid method is robust and insensitive to variations in excitation frequency, which is one of the main sources for errors. We have also analyzed the use of active inductors in our coupled resonators. The analyses and simulations proved the concept. This opens an avenue towards implementation of WCR-based readout in integrated circuits; specifically applicable for micro-electro-mechanical systems (MEMS) devices, and even integrating both MEMS sensors and the readout circuit in the same integrated circuit (IC) package. Another route on this research was to exploit the insensitivity and robustness of three-degree-of freedom (3DOF) weakly-coupled resonators to resonant frequency deviations. Analyses, followed by simulations, proved that applying 3DOF WCR in sensing differential capacitance changes does not require frequency tracking, yet has the same sensitivity achieved in 2DOF-based readout circuits.
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