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Today, I was experimenting with a basic op-amp integrator circuit (I have a piezo element that I ripped from a buzzer, which I would like to use as a mechanical pressure sensor). So I'm using an integrator circuit to integrate the voltage signal produced by the piezo to get a pressure magnitude reading. The circuit is put together using an LM358 dual op-amp IC with a 1uF capacitor and 1k resistor for the feedback network (see the attached schematic).

The circuit works great for the most part except for one small problem. That is that the zero level of the circuit slowly starts to float more and more negative (rather the input is connected or not). If I press on the piezo, it gives me a nice positive hump and then returns exactly back to the zero point, but if I just leave it sitting there, the line floats down more and more until the output signal hits the negative rail (in my case, I'm powering it with +- 5V).

What I'm wondering is if there is a way to compensate for the floating problem to keep the signal stable? I'm guessing that it has something to do with the finite offset difference between the + and - input signals but I'm not sure what to do about it.

Any input greatly appreciated, thanks!

- Jason O

P.S. I should mention that this floating effect occurs over several seconds to minutes, changing by about -1V every 30-40 seconds on average.

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# Basic Op-amp Integrator circuit question

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