Sub-Femto-Farad Resolution Electronic Interfaces for Integrated Capacitive Sensors: A Review

Capacitance detection is a universal transduction mechanism used in a wide variety of sensors and applications. It requires an electronic front-end converting the capacitance variation into another more convenient physical variable, ultimately determining the performance of the whole sensor. In this paper we present a comprehensive review of the different signal conditioning front-end topologies targeted in particular at sub-femtofarad resolution. Main design equations and analysis of the limits due to noise are reported in order to provide the designer with guidelines for choosing the most suitable topology according to the main design specifications, namely energy consumption, area occupation, measuring time and resolution. A data-driven comparison of the different solutions in literature is also carried out revealing that resolution, measuring time, area occupation and energy/conversion lower than 100 aF, 1 ms 0.1 mm2, and 100 pJ/conv. can be obtained by capacitance to digital topologies, which therefore allow to get the best compromise among all design specifications

Analog interface based on capacitance multiplier for capacitive sensors and application to evaluate the quality of oils

Currently, interdigital capacitive (IDC) sensors are widely used in science, industry and technology. To measure the changes in capacitance in these sensors, many methods such as differentiation, phase delay between two signals, capacitor charging/discharging, oscillators and switching circuits have been proposed. These techniques often use high frequencies and high complexity to measure small capacitance changes of fF or aF with high sensitivity. An analog interface based on a capacitance multiplier for capacitive sensors is presented. This study includes analysis of the interface error factors, such as the error due to the components of the capacitance multiplier, parasitic capacitances, transient effects and non-ideal parameters of OpAmp. A design approach based on an IDC sensor to measure the quality of edible oils is presented and implemented. The quality relates to the total polar compounds (TPC) and consequently to relative electrical permittivity ${\varepsilon }_{r}$ of the oils. A measurement system has been implemented to measure the capacitance of the IDC sensor, which depended on ${\varepsilon }_{r}$. The simulation and experimental results showed that, for a capacitance multiplication factor equal to 1000, changes of 3.3 µs/100 fF can be achieved with an acceptable level of noise, which can be easily measured by a microcontroller