Hi, I'm building a Piezoelectric (PVDF) sensor based BCG measuring circuit that will be used to extract HRV and RR. It is supposed to lay underneath a mattress, and measure these using the recoil in our body due to heartbeat and also our breathing rate. The sensor I'm using is the SML-300. No exact datasheet to be found, the internal Capacitance is 2nF.

Ive attached the image of the circuit i made. Small difference, Im not using the VREF rail since i made that because the STM32 cant handle negative voltages but for now im using a Saleae logic analyzer to measure voltage change.

Pressing the fully insulated film (tape + silicone strip) → absolutely no change in the output. No spike, nothing.
The only time I get a response is touching the bare exposed electrode with a finger — and that's a clean 50 Hz waveform, i.e. mains coupling through my body into the high-Z node, not a mechanical signal.
So I have no evidence of any real mechanical piezo response — everything I've seen appears to be electrical mains pickup via bare contact. Ideally a press through the insulator should generate charge and show a spike, and it doesn't.
A second oddity I can't explain: increasing R4 (the gain stage's input-to-VREF resistor, 1k → 100k → 1M) makes the decay transient look more prominent (see the two attached traces). By the non-inverting gain formula (1 + R5/R4) increasing R4 should reduce gain toward unity, so I'd expect the signal to shrink, not grow. Not sure if I'm seeing a time-constant/settling effect rather than real gain, or something else. Image 2 is with R4 = 1K ohms and image 3 is with R4 = 1M ohm.

1. Why does increasing R4 (which should lower the gain) make the transient more visible, not less? Time constant / DC settling effect, or am I misreading the topology?
2. Bigger issue: why would a mechanical press through insulation produce zero output while only bare-contact 50 Hz shows up? Does this mean my sensor isn't mechanically coupling/working, or is gentle finger pressure just too weak/slow to generate detectable charge through a charge amp with a 0.17 Hz HPF?
3. For bare PVDF on a high-Z charge amp, is a grounded foil shield (insulated from electrodes) the standard fix for the 50 Hz pickup? Any grounding gotchas?
4. How do people bench-test PVDF mechanical response in a way that isn't contaminated by body-coupled mains? (Battery power, Faraday cage, mechanical tapper, signal injection?)

Any help is appreciated! Thanks!

 

It might help if you explain some of the terms you are throwing around, particularly all of the acronyms.

 

It might help if you explain some of the terms you are throwing around, particularly all of the acronyms.

My apologies!

PVDF is the type of Piezoelectric sensor ive used (stands for Polyvinyl DiFluoride if im not wrong)
HRV - Heart Rate Variance, basically the time in between two heart beat pulses (J peaks on the waveform)
RR - Respiratory Rate, the rate at which we breathe.
BCG - Ballistocardiography
VREF - Reference Voltage
SML300 - the model of sensor im using, made by Legact film sensors.

Please let me know if any more information would be helpful! thanks!

 

im using a Saleae logic analyzer to measure voltage change.

Can it handle small analogue signals? The name implies it's for measuring digital signals, not analogue ones.

 

Can it handle small analogue signals? The name implies it's for measuring digital signals, not analogue ones.

yes, it has an analogue mode, it can measure voltages as low as 1mV

update: i probed at U3A Pin 1 (the output of op amp A inside the MCP602, and i can see voltage spikes when i tap the sensor kept inside a silicon insulator sheet. so the sensor is fine, so is the software. The issue is the connections around the MCP then I assume. also another issue, yes it's showing voltage spikes due to pressure when i probe Pin 1 but when I zoom closer, the whole thing is once again overshadowed by the 50z mains hum. Do you see any issue with the circuit schematic, connections and parts wise?
thanks!