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TLV2772: TLV2772

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Replies: 8

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Part Number: TLV2772

Hello,

 

I am looking for an appropriate charge amplifier for a Piezo sensor.

The sensor is a PVDF foil, metallized with gold with a size of ~5x5mm2.

The thickness is 28µm (also available are 52µm and 110µm, but I guess we will use the 28µm one for reasons of flexibility).

 

The foil specifications are as follows:

Approximate Piezo Coefficients:

d31 = -5 to -6, d32 = -4 to -5, d33 = -28 to -32,

Di-electric Constant (Relative permittivity):

10-12

Young Modulus~8GPa.

So the capacity of the sensor is roughly ~100pF.

The foil will be incorporated into an artificial finger, basically a silicone form of a real finger, to sense the vibrations which are formed when the finger is rubbed across a surface.

The electrodes are glued to the sensor. The amplifier can be located the closest roughly 5 to 10 cm away from the sensor.

The frequencies of interest, and mainly generated, are between 10 to 2000Hz.

The amplitude of the vibrations, responsible for the induced strain in the foil, will be small however I don’t know how small.

So unfortunately I am not yet sure about the generated voltages but I guess they could be in the mV range.

 

For a start I thought to connect the amplifier (about which I hope to gather some information here) to a USB oscilloscope (Picoscope 2206) with Input characteristics

BNC, 1 MΩ ±1% ∥13 pF ±1 p

This is a 8bit System with a 50Mhz bandwidth.

However the goal is to connect it to an accurate AD converter and connect it to the PC without the use of this oscilloscope.

In  the TI Application report: Signal Conditioning Piezoelectric sensors,  the OP Amp TLV2771 is used for a moderate amount of amplification.

There is a post by Mandar Kothavade: selecting opamp for sensing piezo generated voltages.

Where his piezo has capacitance near about 5nF and it generates voltage waveforms of frequency around 500kHz in the range -5mV to 5mV. and he asks for a opamp which should be able to sense this voltage and amplify it in the voltage range 0-3V.

The suggestions was to use the OPA2301.

However this might be mainly due to the high frequeny demand!?

I am also interested in how to dimension the feedback resistor and capacitance, as for expected small input voltages from the piezo and the desired low frequency cut off of about 10Hz i wonder wheter this can be realized with one amplifier stage or not?

Unfortunately have a confined knowledge when it comes to electronics and so I would be very thankful for any suggestions concerning an approriate amplifier model and tips on the required crcuit!

 

Thanks a lot to everyone in advance!

Best regards

Thomas Ules

  • Hi Thomas,

    Thanks for the detailed post and information, I'm pulling together some material to help you out with your question. I have a couple questions on your application that would be helpful for me:

    1. For input signal, you believe that piezo will provide a signal on the order of mV? Do you know roughly how much force will be applied to the foil?
    2. Do you have any constraints on your supply voltages for the op-amp (single supply of 5V, dual supplies of +/- 5, etc.)?
    3. What is your desired output signal amplitude?

    Thanks,
    Paul

  • In reply to Paul Goedeke:

    It also turns out that there are two app notes published by TI with the same title "Signal Conditioning for piezoelectric sensors." If you haven't seen this one by Eduardo Bartolome, it is also worth your reading as I believe it goes a little more in depth:

    www.ti.com/.../slyt369.pdf

    -Paul
  • In reply to Paul Goedeke:

    One last question, also related to the sensor:

    Do you know the sensitivity of the sensor you are using (typically given in pC/G).

    Thanks!
    Paul
  • In reply to Paul Goedeke:

    Dear Paul,

    thanks a lot for your quick response!!

    unfortunately I am quite uncertain about the amplitude of the Piezo signal as I am so uncertain about the strain induced in the Piezo foil due to the vibrations. I get values in the mV range up to tens of volts and more using the equation

    V=-g x strain x YM x t

    depending on the estimate of the strain.

    Numerical simulations report vibration amplitudes in the range of 100µm. If this deflects the foil I guess the strain could be in the range of about 10-3 which would yield voltages of a few volts rather than milli Volts.In the end the foil will follow the vibrations in the finger sillicone form induced upon friction when rubbing a surface!

    When I get the foil i think i should be able to sense, at least the high frequency, vibrations with the oscilloscope, however this might take a while.

    Concerning the supply voltage I think single supply would be easier as we have single lab voltage supplies however dual supply would also be possible I guess!

    The output signal just needs to be big enough to be clearly discerned from noise i.e. the SNR must be good enough. There is however no need for a minimum value as it does not have to trigger any other source if this was the question? The goal is to measure frequency of the induced vibrations and a value which represents the amplitude of the vibration and be able to do a proper analysis of the obtained signal.

    Thanks again!

    Best

    Thomas

  • In reply to Paul Goedeke:

    Hy,

    the sensitivity is d31 = -5 to -6, d32 = -4 to -5, d33 = -28 to -32 pC/N

    Thanks

    Thomas
  • In reply to Paul Goedeke:

    Hy Paul,
    thanks for sending me the link to the PDF of Eduardo Bartolome. I read this already and amongst other things it evoked the concern about a single amplifier stage as a low feedback capacitor increases the gain but also increases the low cut off frequenciy which migth be problematic concerning the low frequencies I want to measure.
    I also downloaded the TINA TI which is a very useful tool, however the circuit I implemented does not yet work properly...
    Best
    Thomas
  • In reply to Thomas Ules:

    Hi Thomas,

    Once you know more about your sensor and the type of input and output signals you need, we'll be able to provide more specific recommendations and design guidance. For now, I'll cover some of your questions regarding circuit topology and component selection, hopefully that will help you get started on your design. In this discussion, I'll assume that you have roughly 10mV input signals and desire 3V output signals at full scale.

    In selecting the op amp:

    • We can select a low voltage op-amp (Vs <=5.5V).
    • A CMOS or JFET amplifier is best choice due to low input bias current and low 1/f noise corner.

    Designing the circuit:

    • Choose a high Rf - this reduces noise.
    • The circuit needs a gain of 300 or about 50dB. A two stage design is likely going to be easiest to implement.
    • If noise is a factor, consider low pass filtering the output for better results.
    • Cf = 1/(2*pi*Rf*f_low)
    • Ri = 1/(2*pi*C_sensor*f_high)
    • Design a second gain stage depending on gain required - noninverting topology is a good place to start. You may also need to filter depending on sensor resonances and noise.

    Here is an example TINA schematic that shows a possible design for your scenario. The TLV6741 was chosen for its low noise, bias current, and CMOS architecture.ChargeAmp_TLV6741.TSC

    I hope this helps, please let me know if you have further questions.

    -Paul

  • In reply to Paul Goedeke:

    Dear Paul,

    thanks a lot for your help!!

    I will consider your suggestions and will try with the TiNA program a few designs and look up the suggested OPAmp while waiting for the sensor.

    I hope to get the PVDF foil within the next two weeks to get some Knowledge on the Signal Amplitude.

    Again, thanks a lot for your help!!

    Best

    Thomas

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