(OBW0559) lF charge amp, polymer piezo, managing unwanted pyroelectric effects

Thread Starter

Murrayatuptown

Joined Dec 6, 2017
5
Mr/Dr OBW0549 or anyone else...question at end...

MSI tech manual & other polymer sensor literature mention very strong pyroelectric effects...potentially volts/degree C.

With a voltage amp, AC-coupling would probably manage this. I assume most temperature change is gradual and thus at very low frequency...let’s say <1 Hz. But suppose a cold sensor left in the car is brought indoors. Maybe a large temperature gradient can occur at times.

For medium-large sensors (say 1000-20000 pF), and a desired frequency response in the audio range, and a stubborn goal of using a charge amp, I have not only op amp bias and offset to watch out for, but this possible pyroelectric nuisance. I don’t know what kind of a monkey wrench a coupling capacitor presents to a capacitive sensor& charge amp, so I stubbornly forge ahead with my DC -coupled charge amp idea.

I have been reading about servos for DC accuracy. I think that’s the term...may not need such accuracy, but it sounds like a possible solution to control both bias/offset effects and this pyro effect.

This may add another level or two of complexity and interaction with the LF response.

On the one hand, this seems overly complicated, but on the other If I don’t plan on ‘managing’ the unwanted DC ‘drift’ and didn’t know LF pyro behavior existed, I could have problems later that are too late/large to address.

With a feedback capacitance close to the sensor’s capacitance (low charge gain), the feedback resistor need not be as huge as for small sensors (probably 1-10 M ohm). This is better for unwanted DC effects than small sensor and Rf on the order of 1 G ohm.

Am I overthinking this? There’s no harm in overkill as it’s not a commercial idea (my time is free and I could be doing worse things with it). I’ll learn something along the way (hopefully not ‘don’t do that again’).

Maybe a servo with selectable time constant (1-10-100 s) to experiment with how much ‘separation’ from desired signal prevents problems? Lay some great plans, and apply whatever bandaids I left room for). Or maybe a large (100x sensor capacitance) DC-blocking capacitor in series with sensor has negligible divider effect on the desired signal.

The desire to use a charge amp to capture the piezo signal was rooted in the thought all the charge signal is conserved (maybe only with the non-inverting op amp form.

Thanks for reading.
 

OBW0549

Joined Mar 2, 2015
3,151
MSI tech manual & other polymer sensor literature mention very strong pyroelectric effects...potentially volts/degree C.
As far as I know, pyroelectric effects are present in all, or nearly all, forms of piezoelectric accelerometers-- polymer, ceramic, and (I think, but am not sure) quartz.

I don’t know what kind of a monkey wrench a coupling capacitor presents to a capacitive sensor& charge amp, so I stubbornly forge ahead with my DC -coupled charge amp idea.
None, to my knowledge, provided the capacitance of the coupling cap is much larger than that of the sensor.

Am I overthinking this?
I think so.

Some good references on the subject are here, here and here.
 
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