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Amplify signal from piezoeletric

Audioguru

Sep 24, 2016
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Then the output capacitor on the opamp circuit feeds the input resistance of the Arduino. What is its resistance? The Arduino input might already have a series capacitor that does not pass your extremely low frequencies.
 

Diogo

Jun 28, 2017
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On the datasheet of the ADC:
"The analog input resistance is claimed to be 100 Mohms.
During an actual sample, the input resistance is temporarily a lot lower as the sampling capacitor is charged up so it is recommended that whatever you connect to the A/D have an output impedance of 10k or less for best accuracy."
 

Audioguru

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There you go, 10k ohms. Then the capacitor feeding it must be 26uF for a cutoff at 0.66Hz.
 

Diogo

Jun 28, 2017
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Why 10k and not 100M ohms?

52f3bddece395f19378b4567.png

And in the case of using an oscilloscope for testing?
 

Diogo

Jun 28, 2017
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Hi!
So, I have tried to implement the circuit! I don't believe I got 100% perfect results, but I got results, to which I owe you 100% :)!
This is my general input signal (disconnected to amplification circuit):
upload_2017-7-5_19-13-42.png
With a gain of 3 or 6 (can't remember exactly) I got this:
upload_2017-7-5_19-14-43.png
When I see the input signal it also seems to attenuated.
Only setting the gain to 30 I got to see decent amplification (but with lots of noise)!
The circuit I used was the following:
upload_2017-7-5_19-27-33.png
(I started with C1 = 2.2uF but changing to 10uF got me better results.. with 2.2uF I couldn't barely see cough signal! only when i tapped the piezo with my finger!)
Any hints? :|
 

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Audioguru

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You do not show the value of R1. If it is 330 ohms then 48Hz is reduced by by -3dB and 4.8Hz is way down by -20dB.
A piezo is very sensitive to its high frequency resonances. It produces a strong low frequency signal when it is tapped with your finger. Tap it every time you cough.
 

Diogo

Jun 28, 2017
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You do not show the value of R1. If it is 330 ohms then 48Hz is reduced by by -3dB and 4.8Hz is way down by -20dB.
A piezo is very sensitive to its high frequency resonances. It produces a strong low frequency signal when it is tapped with your finger. Tap it every time you cough.
Sorry! R1 was always 100k!
 

Diogo

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The piezo "loading" with 100k (R between non-inverting input and 10uF) can't be causing the killing of the low frequencies?
 

Audioguru

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A piezo transducer has such a horrible frequency response that I have never used one as a speaker or microphone.
Since your opamp has no part number then we do not know if it has a very high input impedance and low input current so the 100k bias resistor can be increased to 1M or higher. Then you could try it.
 

Diogo

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Alright! Will try it later :)

After this amplification stage, will this full-wave rectifier work?
 

Audioguru

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Which opamp part number and what is its supply voltage?
You did not attach a schematic of a full-wave rectifier circuit.
 

Diogo

Jun 28, 2017
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Which opamp part number and what is its supply voltage?
You did not attach a schematic of a full-wave rectifier circuit.
Sorry!
upload_2017-7-6_18-8-18.png
I have made this circuit for an eletromyography circuit and it worked nicely :)
Using OP07
 

Audioguru

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The bias current for the PO07 opamp is very low so you could try a 1M bias resistor to see if the low frequencies from the piezo are better. I have never tested that fullwave rectifier circuit.
 

Diogo

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U mean R1?
If you have a Full-wave rectifier you know ir works, I can use it
 

Audioguru

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Yes, try 1M as R1 in the preamp circuit.
Your preamp uses a single supply but your rectifier needs a dual polarity supply. Why don't you use a single opamp with a single supply for the fullwave rectifier? I use an MC33171 opamp that is a single opamp similar but better than an opamp in an LM324 or LM358. You could try a dual MC33172 opamp as the preamp plus rectifier if its noise in the preamp is not a problem.
 

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Diogo

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Yes, try 1M as R1 in the preamp circuit.
Your preamp uses a single supply but your rectifier needs a dual polarity supply. Why don't you use a single opamp with a single supply for the fullwave rectifier? I use an MC33171 opamp that is a single opamp similar but better than an opamp in an LM324 or LM358. You could try a dual MC33172 opamp as the preamp plus rectifier if its noise in the preamp is not a problem.
Sorry, you mean only try the circuit you attached? With the R1 in that schematic R1=1M?
Will it work LM358 for testing only? (I have it on the lab)
 

Audioguru

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The fullwave rectifier circuit I posted should be fed from a low resistance to ground signal source like from a preamp opamp powered from a dual polarity supply. If an LM358 opamp is used for the fullwave rectifier and its input is capacitor-coupled from the output if the preamp then the input bias current of the LM358 will cause its input to float high and it will fail to work properly.
 
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