Hello dear EEVblog-Community,

I'm currently designing a charge amplifier for a piezoelectric force sensor. I found a lot of material about charge amplifiers so I feel comfortable with my design.

But since I'm a cautious person, I'd like to simulate it with LTSpice and confirm it's behavior.

Here is the data-sheet for the sensor:

https://www.kistler.com/files/document/000-630e.pdf?callee=frontendIt has a sensitivity of -3.7pC/N and a capacitance of 65pF.

I want my amplifier to produce an output of 1V @ 10kN and 0V at 0N.

For the modelling I went with a voltage source with a 65pF capacitor in series, since it seemed like the easiest way. From my understanding I need to calculate the voltage for the voltage source by dividing sensitivity with capacitance of the sensor. That gives my 56.9mV/N. I went with an AC-source that oscillates around 284.6V with +-284.6V amplitude (like a signal from 0N to 10kN would do). I choose a 500Hz frequency.

For the feedback-capacitor I choose 39nF, since that should produce roughly the desired output of 1V at 10kN output. (10000N * 3.7pC/n / 39nF = 0,9487V).

I calculated the high-pass freq as 1 / (2*pi*1GOhm*39nF) = 0,0041Hz

Low-pass freq without cable is calculated as 1 / (2*pi*1kOhm*65pF) = 2.45MHz

Here is a picture of the circuit:

[attach=1]

And here is what I get as a result of the Simulation:

[attach=2]

Am I missing something? I Expect V_OUT to be a sinusoidal curve bewteen 0 an d 1V. Is there a problem in my circuit?

Any help is greatly appreciated

greetings

johofz