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Piezoelectric sensor signal conditioning

Question asked by petermeter on Apr 14, 2011
Latest reply on Apr 30, 2012 by bhuviram



I need a piezoelectric signal conditioning circuit with the highest possible dynamic range to feed an AD7765. The piezo sensor has a high leakage resistance of 20 GOhm. The signal has first to be impedance converted by a charge amplifier or an instrumentation amplifier with an extremely high input impedance/low input bias current. I expect the latter to do a better job because of their inherently high CMRR to reduce noise from the environment. The AD8220 seems to be the right part for this. The sensor is to be connected to the inamp via an unbalanced coaxial  cable and BNC plug/jack. The maximum signal range of interest is +/- 5V but the sensor can  supply more, so the input voltage range has to be limited. Furthermore, a DC return path, input transient and RFI protection has to be provided. The low impedance single-ended output of the inamp is to be converted to  differential and level shifted with the built-in diffamp of the  sigma-delta ADC.


Questions regarding the AD8220 usage:

1.What is best choice regarding analog grounding: connect shielding of BNC jack to AGND or run both wires differentially from BNC jack to -IN,+IN?

2.Knowing from absolute maximum ratings that the part does not tolerate voltages beyond the +/- supply rails, a bidirectional TVS (see attachment) is to limit the input voltage. E.g: working peak: 5V, min.breakdown: 6V. What minimum +/- power supply rails to the inamp do you recommend?

3.For RFI protection -instead of an RC low pass filter- a 3 terminal capacitor (aka x2y capacitor) is planned. Is the correct sequence: BNC,TVS, DC bias resistors, 3 terminal capacitor, inamp?

4.Having seen the example PCB layout of AD8220 datasheet, do you have any additional info on how to place/layout TVS, dc bias resistors, 3 terminal capacitor?

5.In the light of the high dynamic range (>100 dB) any other remarks/suggestions?


Thanks for any feedback!