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TLV2711: Circuit to interface with piezo disc sensors

Jonathan Geronga
Expert 7930 points
Part Number: TLV2711
Other Parts Discussed in Thread: OPA337

Hi Team,

I only use the device for attention but it is part of the document the customer is looking at.
I would like to get your help with this request of the customer for their application.

They are working on a piezo that measures the plantar pressure i.e it measures the pressure at different points on the foot of the person and they are looking for solutions that interface the piezoelectric sensor. Since piezo can output large amplitude (in volts), they need a solution to interface it with the microcontroller.

They've gone through this document below and since the piezo sensors give an output of +-100 V in their application, does the solution might have from TI shall include this?
https://www.ti.com/lit/an/sloa033a/sloa033a.pdf 
https://www.ti.com/lit/an/slyt369/slyt369.pdf


Please let me know if you have any questions with the customer.

Thanks,

Jonathan

  • 0
    TI__Genius 15410 points

    Hi Jonathan,

    The circuit configuration shown in "Signal Conditioning for Piezoelectric Sensors" is pretty standard circuit configuration for amplifying the signal from a piezo. However, it is not immediately clear to me what your request is. Are you looking for  a device recommendation? Or maybe another piece of content?

    Thank you,

    Tim Claycomb

  • Former Member
    0 Former Member

    Hi Jonathan,

    Do you happen to have the data sheet for the piezo sensor/transducer?  If so, could you share a link?

    Regards,
    Daniel

  • 0 in reply to Former Member
    TI__Expert 7930 points

    Hi Tim and Daniel,

    Thank you for your support in this thread.

    Let me first get the datasheet of the piezo sensor the customer is using and I get back to you.

    Regards,

    Jonathan

  • Former Member
    0 Former Member in reply to Jonathan Geronga

    Sure Jonathan.  Please do share when you can and let us know if you have any questions in the meantime.

    Regards,
    Daniel

  • 0 in reply to Former Member
    TI__Expert 7930 points

    Hi Daniel,

    Since they are developing, they are using three piezo discs but all of the same type with only dimension differences.
    Type SP -5A (PZT) ,Piezo disc
    1. 15mm Dia 1mm Thickness
    2. 15mm Dia 0.5 mm Thickness
    3. 10mm Dia 1mm thickness.

    Here's the datasheet family:
    Sparkler Brochure 2019.pdf

    Please let me know if we have something to offer to the customer.

    Regards,

    Jonathan

  • Former Member
    0 Former Member in reply to Jonathan Geronga

    Hi Jonathan,

    Thanks for sharing the data sheet.  Unfortunately, after looking through the data sheet it is not immediately clear to me how the customer came to their +/-100V specification.

    How are they modeling the piezo sensor and do you know how they came to this large voltage?  I believe the piezo amplifier is normally considered as a charge amplifier rather than as a voltage amplifier.

    Regards,
    Daniel

  • 0 in reply to Former Member
    Prodigy 10 points

    Hi Daniel,

      Thank you for your support.

    In fact, your point about high voltage is right and we do need a charge amplifier for our application. We need charge amplifiers that can convert charges from range 450 pico Columbs to 10 micro Columbs to voltage. So, the dynamic range of this amplifier needs to be around 10 e7 wrt charge. The input frequency is below 1Khz.

     I understand the offset voltage, bias current, Noise params are important parameters to consider while we design the charge amplifier. Are there any other considerations?

    The boards will work in +3.3V or 5V and in no way we can have amplifiers with a high voltage supply.

    We went through your products and found that  OPA337 is used for interfacing with Piezo. 

     I hope you have now better informed about what we are looking for and we would like to know your feedback or considerations that need to be taken.

    With Regards

    Sreenath

  • Former Member
    0 Former Member in reply to Sreenath SV1

    Hi Sreenath,

    I am happy to help.  Hopefully you have found some of our online resources to be useful.

    It looks like your limiting factors will be your supply voltage and your maximum charge.  So, I would begin to design the circuit based off of that.  Using the gain formula: Vout/Charge = 1/Cfb, you can determine that for a maximum output voltage of 5V (supply max) and maximum charge of 10uC, a good starting point would be a 2uF feedback capacitor (quite large).  You can then pick your feedback resistor and input series resistor to set the proper high pass and low pass frequencies, depending on your frequency of operation.

    As far as op amp specifications, I would say that the input offset voltage is probably not critical.  But, this depends on the minimum charge at the input.  Given a 2uF capacitor and a 450pC input, that would correspond to an output voltage of just 225uV.  This will be difficult to detect without a pretty accurate amplifier, but this will come at increased cost.  So, this will be a performance vs cost tradeoff.  I would agree that the bias current needs to be small, especially given the large size of the feedback capacitor.  Try something in the range of the low pA if not fA range.  Noise can also become an issue because you may have to use large resistors for your low frequency application and larger resistors make noise issues more challenging.

    These are the ideas I would use to start with.  You can refine and debug problems from there.  I recommend using the parametric search tool on ti.com to begin your search for an appropriate part.  Please let me know if you have further questions.

    Regards,
    Daniel

  • Former Member
    0 Former Member in reply to Sreenath SV1

    Hi Jonathan and Sreenath,

    Since we haven't heard back from you in a while, I hope your questions were sufficiently answered.

    Please let me know if you have any follow up questions or comments.  Else, I will consider the matter to have been resolved.

    Regards,
    Daniel

  • 0 in reply to Former Member
    TI__Expert 7930 points

    Hi Daniel,

    I will click the resolve button once Sreenath has no more questions in this thread.

    Thank you very much for your support.

    Regards,

    Jonathan

  • Former Member
    0 Former Member in reply to Jonathan Geronga

    Hi Jonathan,

    Since it's been nearly two weeks since Sreenath's latest question, I'm going to close the thread for the purpose of internal tracking.

    However, you are always welcome to come back to this thread and ask another question.  If some time passes and thread becomes locked, you can always ask a related question and we will be happy to help.

    Regards,
    Daniel