A simple analog interface for capacitive sensor with offset and parasitic capacitance

In this paper, a simple but efficient analog interface circuit for floating capacitive sensor is presented. In most of the capacitive sensor, the offset and parasitic components are associated with the sensor and varies with the measurand. In these sensors, the change in the sensor capacitance due to measurand can be very small but having a relatively large offset capacitance in parallel and also the value of parasitic capacitances are not constant and varies with measurand. Hence, the error due to offset and parasitic components will propagate in the output which will degrade the performance of the sensor system. The proposed analog front end circuit accepts these types of sensors and provide an output voltage proportional to capacitance change due to measurand. A prototype board has been developed and tested for different values of the parasitic and offset capacitance by varying the variable sensor capacitance. The circuit validate the theoretical expectation and without any visible effect on output voltage on offset and parasitic capacitance. The worst case error for all taken cases are found to be within 1%. In order to test the validity of proposed interface circuit in real time application, a capacitive humidity sensor is fabricated and tested with the proposed circuitry. The interface circuit found to be working well with humidity sensor in the range of 30 to 90% RH.