Connecting a piezo sensor to audio board

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James2017

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I'm interested in connecting a piezo sensor (for input, not output) to the audio board (for use with a Teensy 3.2 to do DSP). Would you advise line in or mic?

Also, in theory a piezo can generate a pretty high voltage, even though the current is small. Do I need to protect the inputs, e.g., with a Zener diode?

Thanks!
 
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James2017 said:
I'm interested in connecting a piezo sensor (for input, not output) to the audio board (for use with a Teensy 3.2 to do DSP). Would you advise line in or mic?

Also, in theory a piezo can generate a pretty high voltage, even though the current is small. Do I need to protect the inputs, e.g., with a Zener diode?

Thanks!
Click to expand...
piezo sensors are typically connected using two antiparallel diodes to protect following circuits, followed by a FET for impedance transformation.
Concerning line-in or mic, it may depend on if you need additional gain, but someone else has more experience.
 
I assume you mean two Zener diodes, facing each other? I'd been using one (so one half the signal always shorts), but two makes sense.
 
James2017 said:
I assume you mean two Zener diodes, facing each other? I'd been using one (so one half the signal always shorts), but two makes sense.
Click to expand...

yes, you wanted to clip pos/neg spikes. Any fast low reverse current diodes are OK. I'm using a BAV199 followed by a BF862 FET for my (piezo) hydrophones.
 
James2017 said:
3.3V Zener diodes? I assume that full dynamic range for the mic or line in inputs is 0 - 3.3V?
Click to expand...

I don't recall the dynamic range of audio board, but I assume there is some bias required.
If you connect piezo without impedance transformer, you have to protect to +- Vbias. (i.e <Vmin and > Vmax)
I'm not sure though if a piezo without impedance transformer will work on both MIC and Line-in.
In case you have any amplification before you need to protect the input of the amplification or impedance transformer.
 
Thanks WMXZ. Biasing (ok, all of this electronics stuff) is new to me. I read about biasing op amps last night and I'm not entirely sure I get it. I think the point is to set the neutral (when there is no signal being input) output to about the midrange of possible input to increase the dynamic range to which the op amp is responsive.

For example, if you want to output from -3.3V to +3.3V, but your input signal runs from 0V to 6.6V, you might set the "ground" of the op amp to 3.3V. 3.3V effectively then becomes your new 0V, since the input is compared to the ground.

In this configuration, an input voltage of 6.6V would look like 3.3V to the op amp, and thus peg the op amp output at +3.3V (assume no gain for simplicity). An input of +3.3V would look like 0, and thus would give an output of 0V, and an input of 0V would look like -3.3V.

If your signal is AC, I'm not sure there is a benefit to bias -- you would just cap the +/- voltages so as not to hurt anything.

Do I have this even remotely correct?
 
James2017 said:
Thanks WMXZ. Biasing (ok, all of this electronics stuff) is new to me. I read about biasing op amps last night and I'm not entirely sure I get it. I think the point is to set the neutral (when there is no signal being input) output to about the midrange of possible input to increase the dynamic range to which the op amp is responsive.

For example, if you want to output from -3.3V to +3.3V, but your input signal runs from 0V to 6.6V, you might set the "ground" of the op amp to 3.3V. 3.3V effectively then becomes your new 0V, since the input is compared to the ground.

In this configuration, an input voltage of 6.6V would look like 3.3V to the op amp, and thus peg the op amp output at +3.3V (assume no gain for simplicity). An input of +3.3V would look like 0, and thus would give an output of 0V, and an input of 0V would look like -3.3V.

If your signal is AC, I'm not sure there is a benefit to bias -- you would just cap the +/- voltages so as not to hurt anything.

Do I have this even remotely correct?
Click to expand...

I think you are on the right track.

But recall, that AC coupled signals need a Zero reference, this may be a true 0V or a virtual zero (biased voltage)

You see the biasing resister circuits in Paul's explanation on how to use Teensy ADC (I think as part of both the Web page and the Audio GUI)
 
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