Resonant Impedance Evaluation Methods for Sensors

In sensors using the resonant principle, it is essential to accurately evaluate the phase of the resonant circuit. The advantage is the high sensitivity of the phase to the sensed quantity. For the proposed qualitative resonant circuit, a temperature stable system was used to enable excitation of the resonant circuit. The sensing of the voltage induced in the measuring structure and the current through the resonant circuit is also included. A dedicated AD8302 circuit was selected for phase shift detection, but this has significant nonlinearity in its conversion characteristics in the phase zero region. Therefore, the detected phase was shifted to the region of greatest linearity of 90° in post-processing. Interfering signals from switching power supplies, mains and radio are sufficiently suppressed without affecting the measured signal. The principles of high-impedance measurement at higher frequencies are applied, resulting in the need for special electronics. Therefore, in order to be able to properly sense and evaluate the transimpedance Z, the electrical voltage from the sensor must be impedance decoupled and amplified at the sensing part. The input of the amplifier should have an impedance Zi in the order of 100G and higher, so that it does not distort the sensed electrical voltage output of the sensor output of the sensing part of the flow meter Due to the need for significant amplification, it was necessary to use an amplifier with ultra-low input current and large bandwidth. The output of the proposed setup is a DC voltage proportional to phase, which can be processed by a conventional A/D converter. The proposed resonant sensor assembly has a stable linear response to phase change and allows small changes in measured quantities to be measured in the field.