Hi community,
I'm trying to simulate a charge amplifier, which is connected to a piezoelectric sensor that will be measuring vibrations. Later down the line, it will be connected to a Low Pass filter and an ADC, so only positive voltages. Therefore, a Vref = Vcc/2 will be used in the positive terminal of the amp.
(This post only focuses on the charge amp and piezo part).
The OpAmp that I will be using is OPA330
https://www.ti.com/lit/ds/symlink/opa330.pdfFrequency range: 100 kHz - 1.5 MHz.
Sensitivity of sensor: unknown (until the manufacturer answers the email).
I'm using LTSpice
I have two main questions:1) Is it okay to simulate the piezo sensor as an AC voltage source with a capacitor in series (The value of which is equal to the electrical capacitance of the sensor)?
I've also come across this paper from TI
https://www.ti.com/lit/an/slyt369/slyt369.pdf which uses a transformer to accurately simulate the sensitivity of the piezo.
2) I've done countless variations of values, by trial and error or by using equations, yet I feel like I can't find any values that correctly show the results.
The main source of information is:
https://www.ti.com/lit/an/sboa287/sboa287.pdfC
f = 1/(2*pi*R
f*f
low) =
159 pF - I'll choose an R
f of 100 MOhm and an f
low of 10 Hz. (I selected 10 Hz because if not, C
f would be very low).
R
in = 1/(2*pi*C
sens*f
high) =
107 Ohm - I find this value pretty low.
I'm also confused, because other sources establish the gain as charge Q divided by Cf [(1/Cf)], therefore Cf does not depend on the first equation.
These values look wrong to me, and the simulations don't seem to look nice or change a lot when playing with the values. Where should I get my values from?
Another observation is that the output should oscillate at 0.9 V, yet it's 20 mV lower. This might be due to R
f having a high value.
The transformer setup should give a charge of 0.5 pC/G. Value from:
https://www.ti.com/lit/an/slyt369/slyt369.pdfThanks in advance. Any advice is greatly appreciated.
--------------
Some information about the requirements:
It has to be able to properly measure frequencies between
100 kHz and 1.5 MHz.
The circuit will be powered by
1.8 V.
Here's the information about the piezo sensor:
Sensitivity is not given in the information sheet
Electrical capacitance
0.992 nFCoupling factor: kp 0.62 ; kt 0.47
Resonant frequency (thickness) 8000 kHz
Resonant frequency (radial) 400 kHz
Outer diameter 5 mm, thickness 0.25 mm