How can one measure particle charge using oscilloscope and op amp integrator circuit along with a capacitor and faraday cup. I am unable to figure out how to approach. I have the circuit ready but when I test the faraday cup by touching a separate charged capacitor, the oscilloscope doesn't show any visible change in voltage. How can I use all these components to get a particle charge. My plan is to drop the particles in the faraday cup and get the output voltage change from oscilloscope. Then calculate the charge with the help of rated capacitor. Also, I don't have any electrometer.  

Can I get help with measuring particle charge using oscilloscope ?. Also, how will the voltage waveform look like when oscilloscope detects any voltage ?

enter image description hereI have attached the circuit, maybe you can have a look into it. What I am doing is charging any capacitor (not included in the circuit) with one end grounded (from dc power supply) and then touching the faraday cup. The oscilloscope is not responding to the touch.

Also, when I am turning ON the power for OP-AMP, then even without applying anything to cup, I am getting a voltage at the output. Is this normal ?... Tell me anything in the circuit which needs to be changed or tell me the step by step procedure which can be used for particle charge measurement using the scope.



1 Answer 1


The negative power supply pin of your opamp has to be connected to an actual negative voltage, not to ground. Use one battery for $V_+$ and a second battery for $V_−$.

The amplifier has to be a very good JFet or CMOS opamp. A TL81 Jfet amp will do for simple experiments, but it won't come close in performance to a part like the LMC6001: http://www.ti.com/product/lmc6001. They are not cheap, but you can usually get one or two samples from TI for free on their website. Generally, never buy an expensive electronics component when the manufacturer will give you free samples. :-)

You need a reset switch across capacitor C because even the slightest amount of input bias current will charge it over time and you need to be able to reset the output voltage to zero. Reed contacts make for the best reset switches of that kind. Get a cheep reed relay and you are in business.

You also need to use a very high quality (ideally polypropylene but at least polyester) capacitor.

$R_{in}$ should be made of several resistors in series to prevent a high voltage discharge from sparking across them, which will inevitably destroy your amplifier. You should make $R>1M\Omega $ to protect your amplifier. Using a spark arrestor or small neon discharge lamp at the input will help the circuit to survive, too. Do NOT try to use diodes as protection elements, it won't end well. Most diodes have way too much leakage for this kind of thing and the ones that don't do not protect the circuit enough. Good old fashioned gas discharges used in the right place are far better.

The whole circuit has to be built cleanly and cleaned extremely well after soldering. Any excess moisture and solvent (alcohol etc) has to be removed by heating above 80 degrees Celsius for half an hour or more. Even small amounts of e.g. solder flux will interfere with the function of the circuit. The very best results can be achieved by "air wiring" i.e. no sockets, no breadboards, just the components, wires and a few solder points. The best solder points for this type of circuit can be achieved on clean sheets of thick plexiglass. That material is an excellent insulator. Drill holes into it for teflon breadboard pins like these: http://www.digikey.com/product-search/en?mpart=11071&vendor=36. A dozen will cost you about ten dollars or so, but you will be glad about using them. The entire circuit needs to go into a shielded case that is grounded. It's better to use magnetic steel than aluminum, so a cheap metal food can with a lid (the cookie type) is just fine.

Good luck with your first science grade electrometer amplifier! (As a student I made these for a living and they sold exceedingly well... not many smart grad students in well endowed labs want to mess with this stuff if they can just buy it.).

  • $\begingroup$ Hey thanks a lot for your comment. I have converted unipolar supply to bipolar supply. By the way I am using TL082 op amp right now. Do i need to change this to LMC6001. Although I have ordered LMP7721 op amp. I am making the circuit as explained in this paper iopscience.iop.org/0022-3727/43/16/165301/pdf/… . So will I have to use reset switches ? I have connected the circuit in breadboard. Do this needs to be changed ( if you have any alternative then please share ). I have used 100 Mega Ohm resistor every where. $\endgroup$ Jun 11, 2015 at 16:02
  • $\begingroup$ I am actually struggling to make the circuit by myself. Can you please share your email-id with me,If you don't mind. I can send you all my circuit diagrams. ( P.S. :- Actually I am not from electronics background ). Also will TL082 JFET not work as reed relay ? $\endgroup$ Jun 11, 2015 at 16:07
  • $\begingroup$ @AnjanTripathi: Sorry, but I have a strong policy of staying anonymous on the internet. However, there is a website of someone who is doing all these things right, let me see if I can find it for you. $\endgroup$
    – CuriousOne
    Jun 11, 2015 at 17:14
  • $\begingroup$ I understand your concern, by the way can you reply on my comments. your ideas can be of great help to me. $\endgroup$ Jun 11, 2015 at 17:18
  • $\begingroup$ Sorry about the delay, I was busy today. Here is an example of how to build a good circuit with air wiring, even though here it's a pico-amp-preamplifer: "air wiring", metal shielding can vk2zay.net/article/251. Another example of (surprisingly!) good practices for high impedance design, again for an ion-chamber: qsl.net/k0ff/01%20Manuals/ION%20Chambers/Ion%20Chambers.htm. Search for Paul Balko "Measuring nanoamperes". That's a very nice paper with an integrator and that's how you can do it, too. More than that, actually, you are playing electronics with the big boys! $\endgroup$
    – CuriousOne
    Jun 12, 2015 at 4:07

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