Best approach to amplify piezo (self-generated) A/C signal at the 1mVAC level

Thread Starter

fe7565

Joined Aug 13, 2016
15
I have a piezo sensor that is putting out 1.1mVAC when resting when a Fluke is connected set on AC Volt. It is quite "steady" at 1.1mVAC, and at times goes to 1.1mVAC and to 1.2mVAC.

Above 1.4mVAC reading there is definitely an external trigger going on, and operational readings are expected to take place at the 2-10 mVAC level. Putting aside improving the sensitivity of the sensor itself, what would be the best approach to amplify it for an ADC-read at a minimum 1V scale but preferably a 3V scale of an MCU such as the Nano. Need to measure the signal amplitude (not just exceeding the threshold). So need to amplify it by min. 1000 times at the 1mVAC reading and 300 times at the 10mVAC reading..

Simple OP AMP? Instrumental OP AMP? Any pre-conditioning before the OP AMP?
 

Thread Starter

fe7565

Joined Aug 13, 2016
15
What is the signal frequency?
The sensor is not excited by any circuit, only the Fluke is connected. I just hooked up the Oscope and getting about a 25 khz average and a Vpp of about 50mVAC that seems centered at around 1mVAC. But at times the freq goes to 100khz or below 25khz. At times the mVAC jumps to 300mVAC.
 
Last edited:

Thread Starter

fe7565

Joined Aug 13, 2016
15
I changed the OScope voltage scale and the freq changed to the 4khz range. The Fluke reads slow and that's why the signal seems "steady" at 1.1mVAC. But the reading change from 1.1mVAC to the 2 and 3 mVAC range is always repeatable and matches with the input force. Do not know yet if linear or not.
 

Attachments

Alec_t

Joined Sep 17, 2013
14,408
Are you sure that what you are measuring is actually generated by the piezo and is not merely interference picked up by the wiring? What vibration source are you trying to measure with the piezo?
 

BobTPH

Joined Jun 5, 2013
9,272
What you are looking at is noise. When deciding what amplification and frequency response is needed, don’t look at the noise, look at the signal you are trying to detect.
 

Audioguru again

Joined Oct 21, 2019
6,783
What is the part number of the piezo sensor?
They have a sharp resonant frequency. Ordinary piezo transducers are used with an external oscillator to make a beeper at their resonant frequency of 3kHz to 5kHz and some resonate at 25kHz and others resonate at 40kHz.

A piezo transducer has a high impedance. Its output level is reduced when feeding 30k ohms or less.

A fairly high quality audio opamp (TL071) is usually used at 100kHz or less and can use a gain as high as 200 for a passband up to 30kHz. But with a gain of 1000 its passband is only up to 3kHz.
My Fluke multimeter is labelled <20kHz.
 

schmitt trigger

Joined Jul 12, 2010
954
There are many Fluke meters, different price ranges and performance characteristics.
A Fluke 115 is only specified in the AC millivolts range up to 500 Hz for 1% accuracy, and 1 kHz for 2% accuracy.
I am right now reading it from the manual.
Therefore for the OP, what model are you using?

But as Audioguru mentioned, a preamp front end is necessary. Years ago I built such a device with a INA143 precision differential amplifier from TI, powered by 3 AAA batteries inside a metal case. The probes were shielded. It worked like a charm.
 
Last edited:

Ian0

Joined Aug 7, 2020
10,276
For your first attempt just wire up an op-amp with a gain of 30 or so, and look at the output on the scope so that you can get a better idea of what is going on.
 

DickCappels

Joined Aug 21, 2008
10,246
For signals like this turning on bandwidth limit might help cut down on the noise. BUT it sounds as if you are picking up an electric field, or given the unpredictable frequency, fields.
 

MisterBill2

Joined Jan 23, 2018
19,424
The piezo sensor by itself, does not generate any signal with no mechanical excitation. So that is the ambient mains electrical signal being picked up by the meter leads, or the scope lead.
So the best way to amplify what actually is generated by the Piezo is in a shielded enclosure. And the actual signal will not be constant.
To increase the signal use a low noise high input impedance opamp, such as the TL072 I think that there is an actual application circuit on the TI website. A manufacturer's application circuit is the best place to start, because they really want the thing to work so they can sell lots of parts.
 

Ian0

Joined Aug 7, 2020
10,276
Why don't you use an insulated gate bipolar transistor (IGBT) for you input stage similar to that used in an Electret microphone? It matches the very high output impedance of the piezo element to the much lower input impedance of an opamp. Here is a link if you need more information:
https://circuitdigest.com/tutorial/igbt-transistor
Electret microphones use JFET amplifiers (because you need at least 5V before an IGBT does anything), and if you're trying to run the preamp off a 400V supply, you'd be better off with a pentode valve.
 
Top