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A typical piezoelectric sensor consists of a PZT-5A ceramic material with metallized electrodes on the surface. Use a conductive epoxy at the electrode to connect the sensor to the external cable. The insulating glue bonds the device components to the structure to be tested, thereby isolating the sensor from the ground reference potential. The piezoelectric sheet faces the direction of the expected acceleration. When placed on the target structure, the piezoelectric sheet becomes a simple pressure sensor and accelerometer that produces a voltage proportional to the pressure and parallel to the direction of polarization of the piezoelectric sheet. Piezoelectric capacitive impedance exhibits a large reactance at low frequencies, making piezoelectric sheets and cables susceptible to interference from surrounding electrical equipment and power lines. When the sensor is placed at a long distance, a shielded interconnect cable is required, but even shielding is not completely effective in removing the common mode signal because the conductive surface of the piezoelectric sheet still acquires noise.
One way to extract the sensor signal is to use an instrumentation amplifier that amplifies only the potential generated by the sensor. The amplifier rejects the common-mode coupling noise potential that appears at each end of the sensor.
A typical miniature piezoelectric sensor has a diameter of 0.125 inches and a thickness of 0.0075 inches, which is equivalent to a capacitance of almost 500 pF. If the measurement application needs to limit the dynamic response of the excitation frequency at 10 Hz or lower, the sensor output reactance can reach a range of 10 MΩ. The insulation of the printed circuit board and the ambient humidity make the amplifier input impedance practically limited by almost 10 MΩ.
Care must be taken to select the isolation method and to use the protection potential. An amplifier with an input bias current of picoampere level must be used. Otherwise, the sensor capacitance and the input bias current resistance of the amplifier apply a phase offset to the signal of the instrumentation amplifier. To eliminate protection and complex insulation requirements, the Figure 1 circuit uses an instrumentation amplifier with feedback to measure the sensor short-circuit current instead of the open circuit voltage. The common mode voltage VCM between the sensor and the signal ground comes from the noise source caused by the surrounding stray capacitance coupling.
Current-mode instrumentation amplifier boosts piezoelectric accelerometer performance
The circuit rejects capacitive coupling noise generated by the piezoelectric sensor and its cable.