High frequency amplifiers for piezoelectric sensors noise analysis and reduction techniques

The measurement and analysis of low level vibrations and Acoustic Emissions in components, fabrications and structures, is often accomplished by the utilisation of multiple distributed piezoelectric sensors. The frequencies of interest usually commence in the upper audio range above 2 kHz, but are more typically ultrasonic frequencies (up to 1MHz). Such measurements are intrinsically limited in terms of their dynamic range due to the signal to noise ratio of the overall system (comprising the transducer and front end signal amplification, together with associated interconnection cables). This paper demonstrates that the latest bipolar operational amplifier technologies (rather than traditional FET technologies) can provide a better solution at higher frequencies in these ultra low noise systems, whilst still delivering the high gain bandwidth needed. This paper also presents a comparative noise analysis for the three principle operational amplifier circuit topologies commonly utilised for piezoelectric sensors: (single ended charge amplifiers, differential charge amplifiers and voltage mode amplifiers). The effects of transducer cables and system configuration are also considered from the noise perspective. The theoretical analysis has been verified by practical experiment.