Amplify piezoelectric transducer voltage

Question

I have an audio-band piezoelectric transducer with a 17 kHz resonant frequency (among others). I'd like to use this transducer to acquire from an MCU an FSK modulated audio signal.

When I plug the transducer into the oscilloscope (with a 1 MOhms input impedance) and I observe the FFT of the voltage produced by the transducer, I observe a peak, with an amplitude of ~100 microvolts.

This amplitude is obviously too low for the ADC of the MCU, so I'd like to amplify the signal, but I don't have any idea how to do it.

How would you recommend to do that? For any purpose, transducer capacitance is supposed to be 13 nF at 120 Hz, according to the datasheet.

Thanks!

Answer 1

Those ceramic piezo transducers have an equivalent circuit very much like a crystal. The "13nF" spec (at 120 Hz) is a measure of Co. The piezoelectric properties translate mechanical flexing of the ceramic into mostly voltage, and a little current. Also introduced is Lm, Cm, Rm which are electrical equivalents of the mechanical motion. Lm and Cm is resonant at 17 Khz. Not shown are all the other resonant modes - each will have its own Lm, Cm, and Rm. (all in parallel). Although these motional components can be measured, its not easy and requires a test jig with measurement tools like function generator, oscilloscope.

^{simulate this circuit – Schematic created using CircuitLab}

Shown are some arbitrary component values for Lm,Cm,Rm. They are resonant at 17 Khz., but shouldn't be taken as accurate - many combinations of values are resonant at 17 Khz., one of which will model your transducer reasonably well.

This transducer will generate currents and voltages near 17 Khz. Source impedance varies over a huge range in a very narrow span of frequencies near 17 Khz. You could use almost any microphone preamp as a signal amplifier.
You can also use a high-impedance preamp to emphasize the parallel-resonance mode of the transducer. This preamp does two things: it amplifies voltage from a high-impedance source, and it presents to the Arduino a bias point about half-way between ground and Vref (assuming Vref=Vcc of 5v):

^{simulate this circuit} This amplifier has a gain of almost 100 at 17 Khz., and gain can be reduced by replacing R1 with a smaller value. Be careful to choose a FET or CMOS op-amp for OA1 having a high gain-bandwidth product. It should also be a "rail-to-rail" op-amp having very low bias current. From Microchip, MCP631, MCP633 would work.