Front-end ΔC/C0 capacitive interface based on negative impedance converter

A simple design methodology of the capacitive front-end readout electronics, based on the negative impedance converter, is presented. The concept uses a parallel connection of positive and inverted-negative value capacitors, that allows removing the constant part C₀ from the sensor gain transfer function. The circuit enables integration of an accurate and very sensitive capacitance-to-voltage converter with ordinary discrete operational amplifiers. This is required in experimental scientific instrumentation as well as in the integrated CMOS interface of capacitive microelectromechanical systems (MEMSs) and touchscreens. Circuit description, noise considerations and measured results are presented.