Designing Charge Amplifier - Page 1
Forum section: electronics / beginners

author: mecs
author_rank: Newbie
timestamp: February 04, 2024, 07:11:04 pm
content: 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 OPA330https://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 LTSpiceI 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 TIhttps://www.ti.com/lit/an/slyt369/slyt369.pdfwhich 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.pdfCf= 1/(2*pi*Rf*flow) =159 pF- I'll choose an Rfof 100 MOhm and an flowof 10 Hz. (I selected 10 Hz because if not, Cfwould be very low).Rin= 1/(2*pi*Csens*fhigh) =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 Rfhaving 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 between100 kHz and 1.5 MHz.The circuit will be powered by1.8 V.Here's the information about the piezo sensor:Sensitivity is not given in the information sheetElectrical capacitance0.992 nFCoupling factor: kp 0.62 ; kt 0.47Resonant frequency (thickness) 8000 kHzResonant frequency (radial) 400 kHzOuter diameter 5 mm, thickness 0.25 mm

author: David Hess
author_rank: Super Contributor
timestamp: February 04, 2024, 08:15:59 pm
content: Quote from: mecs on February 04, 2024, 07:11:04 pmSure, that works fine.QuoteThe feedback capacitance controls the high frequency gain. The high value feedback resistor controls the DC gain and something is required to provide the DC bias current into the inverting input; there are other circuit configurations which handle this in a different way.The input resistance is needed because as frequency increases, the sensor capacitance adds phase lag between the output and inverting input, which will eventually result in oscillation or at least large amounts of high frequency noise. The input resistance adds phase lead to maintain stability so at high frequencies. Its value can be derived from a bode plot. Study operational amplifier differentiators to get a better understanding of why the series resistance is necessary.

author: MarkT
author_rank: Frequent Contributor
timestamp: February 04, 2024, 08:51:15 pm
content: Piezo sensors act more like current sources with capacitance. In particular they can develop large voltages in some situations into high impedance loads, so you may want to consider protection circuitry, and carefully chose the load impedance.

author: jwet
author_rank: Frequent Contributor
timestamp: February 04, 2024, 09:32:29 pm
content: A charge amp is an unusual circuit. They are common in photodiode circuits where most writing has been done.The "gain" of a charge amp is 1/feedback capacitor value. The R across the C is to provide some DC bias to the amp and discharging the C with a long time constant. Its often best to think about Q amps in the time domain as a pure integrator with a leaky cap. What you're doing is taking the input charge (coulombs= I x t) and depositing it on the feedback C, creating an output pulse proportional to the charge deposited. If you deposit a picocoulomb of charge on the input, the feedback cap will end up with this on it, it will discharge through the 100M. If the cap was 10 pF, you'd get a tenth of a volt that would be decay with an RC of 1 mS. With a piezo sensor, you probably want to put pair of diodes clamps on the input.The capacitance of your source matters too, looking into a charge amp, the noise gain is 1+ Cin/Cf, this also goes up with frequency and gives rise to "noise peaking"- having noise rise at higher frequencies. "Photodiode Amplifiers" by Graeme covers this somewhat. There is a tradeoff with gain and noise. I've done a little with charge amps for vibration but mostly photodiodes but this sounds different yet. There are sparse references for Q amps. Good Luck.One of the other oddities is that you'd like to make the input node at the negative amp input as a virtual ground and an "infinite" charge sink (a BIG cap). The AC input impedance is a strong function of the GBW of the Op-Amp and will often require a much faster op-amp that you would expect. Any charge desposted that isn't collected instantly is lost- time domain thinking again.Hope this makes sense. Challenging circuits.Good luck.

author: mecs
author_rank: Newbie
timestamp: February 14, 2024, 12:59:32 am
content: Well, turns out I was using an OpAmp that wasn't suited for my frequency range. The values I got from the calculations seem to work correctly.Thanks for all the help and suggestions!