how to make a microphone out of PVDF piezo film ?

Other Hardware Sensors
1

Hi all,

I have a sensor created from PVDF plastic which is a piezo material.

electrically it is like a (plm 15pf) capacitor and when stressed the charge leaks out a bit
just like a sponge. (sounds awfully like an electret mic to me, but apparently there is another principle at work here ?)
This charge is extremely small , hence the current is extremely small (pA range)

as a knock sensor this stuff generates quite a voltage, but as an microphone the voltages are also small

I know the basics about opamp but that does not mean I can design with them (yet).
So what (obtainable) opamp type would be useful as the interface
with this PVDF film ? obviously a JFET type, but which brand & model ?

complicating this is the fact that I want to process signals between 10 and 100 kHz (think bat detctors),
so the obvious audio opamps are often not having enough gain/bw headroom.

I would love to hear from somebody who created a working preamp for a PVDF film (used as microphone)
preferrably with a higher than audio frequency range. I do know about an article in Circuit Cellar, but the
opamp used there seems hard to obtain.

Regards,
Ronald

2

As you say, a jfet (or cmos) op-amp with a high gain-bandwidth product is what you need. A quick search of RS Online revealed a few candidates including Microchip MCP6291-E/P (cmos, works from single 5v supply, 10MHz gain bandwidth product). Or MCP6292 for the dual version.

3

Thx for the suggestion of using RS online.
it took some time but now I can use their selection tools (duh!)
selecting 10 mhz GBWP and Input bias of 1pA got me a few candidates. including one with even lower
input bias : AD8691 (shame about the temperature dependency though..)

I did spell out the MSI manual already, but as said, that does not mean I can design based on it.
I'm looking for a hand to hold and guidance to follow :wink: preferably in the form of proven previous designs...
nevertheless, I think I will revisit that manual just after rereading Horowitz and Hill on opamps.

4

Did you get anywhere with this?