A 15 PPM RESOLUTION MEASUREMENT SYSTEM FOR CAPACITANCE TRANSDUCERS

A new microprocessor-based measurement system using an LC resonance lo cking technique for capacitance measurement is presented. The measurin g circuit automatically finds the resonance frequency and tracks its c hanges, which depend on capacitance changes. The instrument has high s ensitivity and resolution, wide measurement range, fast response time and is insensitive to stray capacitances, due to the shield and the wi ring, and to their variations. Although the measuring system can be us ed in different fields such as laboratory and industrial applications, we designed it as a conditioning circuit for industrial capacitive tr ansducers, such as capacitive pressure or displacement sensors. The ci rcuit showed a response time of the order of 2 ms, a resolution of abo ut 0.1 fF for an 8 pF transducer capacitance, with stray capacitances ranging from 4 to 20 pF, and a sensitivity of about 100 Hz fF(-1) in t he range from 6 to 16 pF. The measurement technique and the circuit ar e described in detail in the paper.