I currently use the Senther 540B sensor to measure the intencity and duration of a shock on a metal surface.
I use the circuit in the first image with an additional 100nF capacitor between REF and GND.

As you can see in the second picture, the sensor measures the shock correctly (I think) but the signal amplitude tends to decrease and reach saturation at the end. After a while, when the shock is over, the sensor returns to the home position. The sensor seems to lack responsiveness or something.

I would like to know if my output signal is correct. Is it normal that the output voltage decreases during the shock?

Thanks for your help !

Hi alex.digonnet43,

Welcome to the Tech Forum!

What amplifier are you using and what supply rails are you using for its V+ and V- rails? It looks like it is clipping on the lower rail, meaning the output of the amp is incapable of going below that level (around 0.9V or so).

It does seem odd that the output would droop during the shock. I’m not sure what might be doing that. Will have to look further into this to figure that out.

Additional question: Does the output signal have the same declining average output if measured directly on Vout of the 540B device (or on the input side of C2)?

So, after rereading your initial post, you said that you added an additional 100nF cap between REF and GND. So you have that cap in parallel with R2?

If so, that forms a low-pass filter in the REF pin. If the nature of your after-shock frequency response tended to be of a higher frequency than the initial shock, wouldn’t that pull the reference voltage lower as the higher frequency content was filtered out?

Edit: On the second thought, since the REF pin isn’t seeing the signal directly, this may not be the case. But it would do this for any noise on the Vcc rail. Regardless, I would remove it, as C1 is already performing the same function.

I tried with 2 different amplifiers but I end up with the same result, but now I use MCP6021. I use V+ = 3.3V and V- = 0V. Yes I agree with you, and this problem can probably be due to my bias (not perfectly placed in the middle of 3.3V). So I think I can solve that problem.

And yes the output signal has the same decline if measured directly on Vout of the 540B.

If you see the shock with more time, the output signal is like on the picture (for a small shock and not the same one on the previous picture but the idea is the same). On this picture the signal is measured in differential between Vout and REF, that’s why it is centered around 0.

Thanks !

Yes tried already to remove this capacitor but the output signal remain the same. I added this capacitor because in the datasheet the schematic regarding ADC use a capacitor.

Can you send an image of your test set-up and circuit board? Perhaps that might provide some clues? I’m trying to think of any physical phenomenon which might cause non-ideal behavior which would deviate from a theoretically ideal circuit. Things like long lead-lengths, parasitic components, power supply impedance issues, non-ideal physical connections, etc. which could affect the results.

In a separate theory, is it possible that the internal piezo structure tends to only deflect in one direction (with initial positive oscillation components) due to the nature of the material it is attached to and how that material behaves when a “shock” occurs? If this were the case, then I believe that physically inverting the whole mechanism (turning it upside down) ought to have the opposite effect. This seems like an unlikely scenario, but I’m just brainstorming.

It might be interesting to see whether you get the same negative output “sag” if you test it on different types of materials/surfaces.