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OPA1678: [opa1678]question on figure35(pvdf case) and negative voltage

River Liu
Intellectual 365 points
Part Number: OPA1678

Hi there,

We are considering using opa1678 as preamp of a pvdf audio sensor. Since there exist silver migration we do not want to put bias voltage to pvdf and thus figure35 in the datasheet is a good start.

1. With above R5&R6 connection, there will be a 1/2Vs+ voltage to bias piezo. 

The pvdf film we are using does not need a bias voltage which leads to silver migration. So is it OK to just remove above R5&R6 to remove bias voltage?

2.Could you provide an example to connect the opa1678 outputs to an audio ADC. We have studied many audio ADCs and they only accept single-end voltage. But in above case I think it outputs negative voltage as well.

B.R.

  • 0
    TI__Expert 6250 points

    Hi River,

    In the above circuit, both terminals of the piezo will be biased to Vs+/2, which is necessary to keep the OPA1678's inputs at an appropriate common-mode voltage (mid-supply in this case). This wouldn't apply a differential DC bias to the piezo element, but if no DC voltage on the piezo element is acceptable, you could add DC blocking capacitors of 10µF or so in series with the piezo element.  

    For a single-ended input, I'd consider the following as a starting point:  

    In this case, U2 - the second half of the OPA1678 - is used to provide a buffered mid-supply reference required to bias both the input and feedback network of U1. C1 blocks any DC bias from reaching the piezo element, R4 bleeds any DC charge off of the microphone, and Z1 and Z2 provide overvoltage clamping to prevent damage to the OPA1678 if the microphone is dropped. U1 is configured for a DC gain of 10x, and its output is again AC coupled through C3 before going to the single-ended ADC. Depending on the ADC itself, it may be necessary to add additional low-pass filtering between this circuit and the ADC - if this filtering is done with an active filter, it may make the most sense to add it between U1 and C3, as a DC blocking capacitor will be required ahead of the ADC itself. 

    Depending on the piezo microphone's characteristics, the gain of this circuit may need to be modified to provide an appropriate input level for the ADC. 

  • 0 in reply to Alexander Davis
    Prodigy 10 points
    Thanks for the solution. Indeed after a lot of study, I have decided to use DC bias on pvdf film. Anyway it is a long time process even if there exists silver migration. And your proposal is also reasonable in no bias condition. I will adopt both solution to have a test.