Gain Adjustment for Signal-to-Noise Ratio Improvement in Electrical Capacitance Tomography System EVT4

A very small value of inter-electrode capacitance is a main factor limiting image spatial resolution of electrical capacitance tomography. The front-end sensing circuit of multichannel capacitance tomography system EVT4 was designed for measurement of extremely small capacitance values in the range from 1 fF to several pF. The capacitance value is determined by charging it using a single high voltage pulse and integrating a charging current. The idea of gain adjustment in the front-end amplifier of a capacitance measurement circuit is presented. The switchable feedback loop of the inverting preamplifier-a first stage of the sensing circuit-enables gain adjustment in a wide range. The measurements using a cylindrical 2-D capacitance sensor were performed without and with gain adjustment. The obtained results showed improvement of signal-to-noise ratio for the pairs of opposite electrodes from about 22 to 30 dB.