Design, Realization and Characterization of a Differential Charge Amplifier for Ultrasonic Piezopolymer Transducers

Low capacitance (tens of pF) piezopolymeric transducers used in SHM systems are usually connected to electronic front-end by cables which parasitic capacitance is comparable to the transducer one. To avoid signal attenuation due capacitance partitioning occurring with high impedance voltage amplifiers, a novel architecture of a fully differential charge amplifier is proposed. The -3dB band of interest for this design is 100 kHz-1MHz. The electronic design carried out by simulations is presented, and then experimentally verified. A 245dB [V/C] measured conversion gain, is particularly suitable for preamplification of very small amount of charge (~ fC), and a theoretical differential input impedance lower than 140Ω on the whole band. A common mode rejection ratio of 60dB and 50dB has been obtained for the variants with active and resistive loads of the differential amplifier respectively.