A simple analog front-end circuit for grounded capacitive sensors with offset capacitance

This paper presents a simple but efficient analog front-end circuit suitable for grounded capacitive sensors. Most of the practical capacitive sensors have an offset capacitance in parallel with a variable capacitance that varies with the quantity being sensed. In some applications, value of offset capacitance can be very large compared to the variable capacitance. This will introduce large offset voltage in the output of conventional measurement circuits. The proposed front-end circuit accepts such type of sensors and provides an output proportional to change in the variable capacitance alone. This increases the overall sensitivity. A prototype circuit has been built and tested for sensors with offset capacitance up-to 1000 pF and variable capacitance of 100 pF. No effect of offset capacitance was visible in the output of the circuit. Worst-case error observed during the measurement was less than 0.05%. Efficacy of the circuit was further tested by interfacing it with a prototype capacitive human proximity sensor. The developed system showed a reliable detection range of about 20 cm.