A novel general purpose current mode oscillating circuit for the read-out of capacitive sensors

In this paper we present a novel general purpose current mode (CM) solution for the interfacing of capacitive sensors. The designed circuit, which utilizes only two second generation current conveyors (CCIIs) as active blocks, allows to detect, through a capacitance-to-time conversion (C-T), capacitive values as well as their variations (also lower than 1pF) both in a small and a wide range. Its main operation is based on a current differentiation, instead of voltage integration, typical of interfaces developed in the voltage-mode approach. The proposed circuit, which does not need any initial calibration, has been designed as integrated solution at transistor level in a standard CMOS 0.35µm technology. Waiting for the chip fabrication, preliminary experimental results have been performed through a discrete-component board and sample capacitors and resistors. Both simulation and experimental results have shown low percentage errors and a good agreement with theoretical expectations for more than five frequency decades. The system sensitivity has been set to about 1.8µs/pF.