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Piezoelectric sensor preamplifier

I have a piezo sensor providing me a 1mV voltage signal on a 1Mohm input impedance measurement instrument. I need to build an amplifier which can return an output voltage of about 1V.

Which do you think would be the best solution: a 2 stage voltage amp: a voltage follower and an inverter, capacitively coupled or a transimpedance amplifier.

To me, it seems the voltage amplifier solution is the best. For a 1mHz to 40 Hz bandwidth which op amp would be most appropriate; what do you think about OP07?

Thanks in advance!

  • Straight forward voltage amplification seems the obvious choice unless there are constraints you haven't mentioned. You are looking for a gain of 1000 at 40Hz, so a single stage amplifier can do the job provided it has a GBW of at least 400kHz. If you really need to go down to 1 mHz, then you will be looking at DC coupling, and you will need a really low offset and low drift amplifier. After all, a 10uV offset is 1% of your signal. I would ignore anything as old as the OP07 (excellent in its day) and look at the ADA4528 or AD8571.

    PS don't forget to add input protection as piezos can generate big voltages if you knock them

  • You need a "charge" amplifier.

    This is nothing more but a VERY low input current opamp and instead of a feedback resistor you have a feedback capacitor in parallel with a 10M or higher resistor (or the thing will not be stable) to the inverted input. The capacitance should be the same as the piezo sensor.

    Absolutely not OP07, the input current is far too high. You are interested in low current noise, not low voltage noise opamps.

    Even so, I do not know what type of sensor you are using but it might be a hydrophone or an accelerometer. Some of these devices have a very low capacitance. If you connect this to a common voltage amplifier which has a significant input current you will severely limit your low frequency response.

  • Thank you all,

    Thus, it seems that OP07 is somehow left behind by the newest (actually better) types of op. amps.The 400KHz minimum GBW as the DC coupling is most useful info. The chopper stabilized op amps are really a peak in low offset and low drift. I still fear a charge amplifier will be needed, since this is the most recommended for high impedance sources but it seems that the long cables somehow succeed to add noise if I use this kind of input preamplification (also an option to my instrument). Actually the feedback resistance would have to be as large as 110 Mohms and it needs special shielding. The low current noise information is very useful. Indeed there are far better op amps from the polarization current point of view.

    It was very helpful for me to receive feedback from actual specialists.

    Thanks again!

    Best regards!

  • And attached is the document containing the ADI specs advised, chosen and tested solution. It is interesting to see it works (pretty well, given the settings). It was good to receiving experienced advice!

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