Low noise amplifiers for piezo type sensors - anyone know of any good applicati

ElectricAye

Hi sensor fans,

I know next to nothing about acoustic emission sensors, which are based on piezo elements, but I've heard they need low noise amplifiers, of which I know even less than nothing. Anyone know of a good application note put out by a chip maker, etc. that might give a newbie a clue? The sensor is an HD-50 by Physical Acoustics (see data sheet attached). In the raw, the sensor puts out about 10 mV or more when I tap gently upon it with a pencil, but I'd like to amplify and maybe filter it so I can interface it with the Propeller somehow.

thanks a heap,
Mark





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Watching the world pass me by, one photon at a time.

Tracy Allen

That will be a high impedance amplifier for the piezo. Take a look at Linear Tech, in particular dn140 design note on choosing op amps for low noise performance. You might choose something like an LT1113, a low noise jfet input op amp. There are example circuits in there for things like hydrophones, which are also piezo devices. Other op amps flagged as suitable in dn140 will have additional circuits. Look for similar parts on other manufacturers' web sites too (National, TI, Analog Devices, That).

A lot will depend on the signal bandwidth and level you need to look at. I see that your HD50 sensor peaks at 500 Hz, but it stays within 20dB of that over a range of about 100Hz to 1kHz with knarly ups and downs. I am puzzled by the reference level that they give, -70dB at 500 Hz ref 1 volt/mbar. How can you reference a sensor meant to be attached to a surface to millibar, a unit that makes more sense for a microphone or hydrophone responding to pressure?

For system level stuff, past the op-amp circuit, look into audio engineering references. For example, a google search for Bill Whitlock (of Jensen Transformers) will turn up a lot of accessible material.

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Tracy Allen
www.emesystems.com

ElectricAye

Tracy,

thanks very much. That's exactly the kind of lead I need.




As for this...

Tracy Allen said...
...How can you reference a sensor meant to be attached to a surface to millibar, a unit that makes more sense for a microphone or hydrophone responding to pressure?....

I am baffled, too, by this sort of reference. Yet it seems like what the acoustic emissions people like to use. Maybe because the wavelength is so small it's more like sound than movement???

thanks again,
Mark

Tracy Allen

It looks like there is also Linear Tech DN355, an update of DN140, covering more recent op-amps.

With this acoustic emissions sensor, they want to fasten it to an object such as a motor to detect vibrations, is that right? Vibrations then might translate into acoustic emissions. The sensitivity of the sensor itself would be in terms of its own internal kinematics, position, velocity, acceleration. Without knowing the structure it is attached to how can they relate that to a pressure sensitivity in units of Volts/mbar? (analogous to the mechanical properties of a capacitance microphone diaphragm, with in internal position sensitivity in Volts/micron, related through the mechanical properties of the diaphragm to a pressure sensitivity in V/mbar)

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Tracy Allen
www.emesystems.com

ElectricAye

Tracy Allen said...
...
With this acoustic emissions sensor, they want to fasten it to an object such as a motor to detect vibrations, is that right?....

Actually, for my application, I hope to listen to ultrasonic crackling noises caused by a material as it is stressed. The material can sometimes emit flashes of light when cracking, and because I hope to monitor light emissions caused by other phenomena taking place at the same time, I want to effectively "blank out" any light flashes caused by cracking of the material. I have no idea if the HD-50 is the right sensor for this; I just got it as surplus to get me started monkeying around with these kinds of sensors.

thanks for the update on the Design Note. This is great stuff.



Mark