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# TLV2771: Drift/Offset

Part Number: TLV2771

So, I got this circuit, my sensor is a piezoceramic disk and I use small round ball of steel to impact the piezo.

The problem  is when i connect the OpAmp the signal is offset and this offset increase with multiple impact.

I just dont know why this happen and how to solve that.

• Hi Jonathan,

Do you get the same effect if you put some time between the impacts? What happens with just one impact? My guess would be that there is some capacitance being charged causing the offset. Can you AC couple the piezo to the signal chain and get rid of the DC information altogether?

Best,
Paul
• In reply to Paul Goedeke:

Jonathan,

This is classic capacitor clamp behaviour.

Note the that maximum negative voltage is the same for both impacts. That's the clamp in action.

You need a DC bleed (bias) resistor otherwise the input can float to any voltage.

Connect the resistor to a positive voltage to prevent clamping the negative peaks.

Regards,
Ronald Michallick
Linear Applications

TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

• In reply to Ron Michallick:

Hi,
I try to put resistor 100M 10M 1M for bleed but the offset is still there, when i shoot 1 ball or many at the same time the result is the same, but on 1 impact the signal look like a capacitor discharge. I measure the Piezo capacitance it is 2.6nF. On the picture I use 1M bleed resistor.

I want to feed the signal the a ADC, I don't want the negative voltage , I only need the positive peak to measure the force of the impact.

• In reply to jonathan tardif:

Hi Jonathan,

the input of OPAmp cannot handle the negative input voltages your piezo is producing. You are violating the common mode input voltage range.

Kai
• In reply to jonathan tardif:

Jonathan,

Do you need a peak detector to hold the peak voltage for the ADC or is the ADC read fast enough to capture the peak in real time?

This circuit will not shift DC bias with strikes.

Regards,
Ronald Michallick
Linear Applications

TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

• In reply to Ron Michallick:

Hello Ron,

I do need a peak detector and a schmitt trigger to measure the first pulse period , but my plan what to put a buffer to isolate the piezo from the rest of the circuit.

I will try your circuit, thanks.

• In reply to jonathan tardif:

Jonathan,

With peak detector and reset. Reset could be replaced with bleed resistor.

TLV2772_PeakDetector.TSC

Regards,
Ronald Michallick
Linear Applications

TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

• In reply to Ron Michallick:

Problem fix, thanks to all of you and i learn many thing.
have a nice day.