Weak-inversion measurement circuit for miniature electrical capacitance tomography

This paper describes the design of the front-end measurement circuit for a miniature electrical capacitance tomography system. This is based on a charge-transfer circuit and is implemented as a custom CMOS integrated circuit that is mounted close to the electrodes to reduce parasitic capacitance. The design has been informed by a 3-D finite-element simulation using the Ansoft Maxwell software. Simulation results suggest that, with averaging, a minimum, measurable capacitance change of 100 aF (10(-16)F) is possible. This paper presents an analytical description of the circuit which, uniquely for such an application, is based on MOSFETs operating in weak inversion. The system has an average sensitivity of 40 mV/fF and can deliver an unprecedented 6250 measurement frames/s. The sensor comprises eight electrodes that are formed on a conical orifice, fabricated using ultrasonic drilling and a minimum diameter of 0.5 mm, through a ceramic substrate. The cross-sectional tomographic images of static objects, which are reconstructed offline, are presented.