How to measure output of a piezoelectric?

Thread Starter

Rori

Joined Nov 23, 2021
6
Hey all,
let me first say hello, as a newcomer on the forum! Good to be along electronic specialists.

I have a (perhaps trivial) question that is buzzing me a bit these days. A brief explanation of the project: I have metallic plate with two piezoelectric sensors attached on each side. One is used as transmitter (TX), other as receiver (RX). I am driving TX with function generator and high voltage amplifier. On RX I am trying to measure the output (see the waveform).

Now, problem is the following: When I am measuring with oscilloscope, it picks up the noise from the power supply, which is generated by the amplifier (noise is exactly the same shape as input signal into TX piezo). I have tried using power transformer to get rid of the dirty ground, but it doesn't change a thing. I know I have to create some 'buffer circuit' between RX and oscilloscope and have some ideas, but would also appreciate your ideas/suggestions, not to spend too much time on this problem.

Here's the circuit I have thought of:
20211118_162223.jpg

INA stands for instrumentation amplifier (classic schematic with three op-amps), lower part is an averager. I would measure with oscilloscope between the two output on the schematic.

Any ideas/thoughts are welcome.

Thank you and best regards,
Blaz
 
A piezo transducer if it is bumped can generate a voltage high enough to destroy an amplifier input. Then you need on or two voltage limiting devices.

Your piezo appears to be floating (neither end at ground) therefore it picks up lots of interference.
 

Thread Starter

Rori

Joined Nov 23, 2021
6
A piezo transducer if it is bumped can generate a voltage high enough to destroy an amplifier input. Then you need on or two voltage limiting devices.

Your piezo appears to be floating (neither end at ground) therefore it picks up lots of interference.
Good point on the voltage limiting. I will take care of it. Thanks!

Regarding the noise. It is floating, but the problem is that noise is introduced to oscilloscope via the power line. How can I get rid of it? Any ideas?
 

KeithWalker

Joined Jul 10, 2017
2,034
An oscilloscope probe will act as an antenna if it is left open circuit. How are you making your measurement between the two outputs? You need a ground reference for the differential measurement. The best way would be to use two channels of the scope, set to the same range, connected to the two outputs with the input mode set to A - B. The ground clips of both probes should be connected to the circuit common. Any interference present at the two probes will cancel out.
 

Thread Starter

Rori

Joined Nov 23, 2021
6
An oscilloscope probe will act as an antenna if it is left open circuit. How are you making your measurement between the two outputs? You need a ground reference for the differential measurement. The best way would be to use two channels of the scope, set to the same range, connected to the two outputs with the input mode set to A - B. The ground clips of both probes should be connected to the circuit common. Any interference present at the two probes will cancel out.
Thank you! Since I have no circuit on the output (and just want to measure solely piezo output), is amplifier's common ground good in this case?
 

Thread Starter

Rori

Joined Nov 23, 2021
6
Hey, I tried, but the problem is that interferences at the probes are not the same - therefore they don't substract. Any other ideas?
 

KeithWalker

Joined Jul 10, 2017
2,034
Hey, I tried, but the problem is that interferences at the probes are not the same - therefore they don't substract. Any other ideas?
What you are measuring is the differential electrical signal across the transducer. It is either picking up radiated EMI or it is detecting mechanical vibrations of some kind. A picture of your scope waveform would be a great help in solving your problem.
 
You are using a mechanical mass based piezo transducer which generates small mV I guess when vibrated for example using a mobile phone small vibration motor. You can add a capacitor of a good value across the transducer to stabilize the voltage fluctuations so that the capacitor charges to a stable voltage and fluctuations either fully stop or reduce very much for easier measurements. Try say electrolytic 10uF 10V, 100uF 10V,... etc,...

For better answer provide the data sheet of the MEMS piezo transducer.
 

MrChips

Joined Oct 2, 2009
24,610
As AG says, you can get a couple of volts from a piezo transducer.
What you are attempting to do is the same as an electret microphone.
Look up how an electret mic is designed using a simple FET preamp circuit.
 
The transducer is a very high impedance device and will be very susceptible to picking up radiated electrical energy. You can confirm whether the interference is appearing at the transducer terminals by making a differential measurement directly across it with the scope probes. Radiated interference will be approximately the same at both terminals of the transducer and should cancel out. Mechanical interference will generate a signal across the transducer and will appear as a deflection in the differential measurement. You should be using shielded wires between the transducer and the circuit, with the shielding grounded at the circuit common. You should also have grounded metal shielding around the transducer to protect it from radiated EMI..
 

Thread Starter

Rori

Joined Nov 23, 2021
6
Fist of all, thank you all. I appreciate your help.

I can post scope picture tomorrow.
I figure out that what is the main source of the signal I detect on the scope is EMI radiation. I need to make sure that I provide shielding to avoid picking up that radiation.

Thanks for the advice on the mic preamp, I will look into it!
 
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