Measuring insect voltage using capacitive coupling and instrumentation amplifier

Hi everyone,

I am trying to measure the potential of some charged object by using a tungsten wire capacitively coupled to it and having this electrode connected to an instrumentation amplifier (namely the AD624). Ultimately, the signal will be converted using an AD converter but firstly, I am working on the sensing and I am observing the output via an oscilloscope.

I've seen this sort of setup in several papers and I am trying to replicate such experiment myself but I am unsure of all the principles and was wondering if anyone has any further advice in terms of the circuit?

I have tested the in-amp using a wave generator and connected the positive and negative terminals to the non-inverting and inverting inputs respectively and got results that is expected. However, when it comes to using the tungsten it becomes more tricky.

I was thinking of soldering the tungsten wire to a piece of wire, which then will be connected to the non-inverting input of the in-amp. The inverting input will be connected to ground.
Does anyone foresee any issues with this?

Essentially to summarize, I want to sense a static voltage on an object (such as an insect hair) and measure it.

Here is a picture of the tungsten needle



Many thanks

there is a fundamental problem here: your amplifier will measure the voltage between the non-inverting input and ground; what you are proposing measuring the voltage between your insect hair and ground. Basically, you need a reference point in your measurement, and I don't think ground is it. Do you want to measure the voltage ACROSS the hair? I think what you REALLY want is to measure charge, and your proposed circuit won't do it.

barry said:

there is a fundamental problem here: your amplifier will measure the voltage between the non-inverting input and ground; what you are proposing measuring the voltage between your insect hair and ground. Basically, you need a reference point in your measurement, and I don't think ground is it. Do you want to measure the voltage ACROSS the hair? I think what you REALLY want is to measure charge, and your proposed circuit won't do it.

Click to expand...

Thank you for the reply!
Let me give some examples. Here is an image of the kind of circuit I am going for.
I am trying to use a similar setup but instead of a KCl sensor, I am using my tungsten wire. The amplifier they are using is also the AD624. They too are capacitively coupling the voltage generated to the electrode.


In the study below, they are using a tungsten electrode stuck into the flower stem. The amplifier they are using is the WPI DAM-50 differential amplifier. They are measuring the potential of the bee as sensed by the flower as it approaches.


Here is how they describe the setup:
"The stem potential is recorded differentially with a reference electrode inserted into the floral foam. The potential difference between the reference electrode and ground is subtracted from that of the measurement electrode."

Basically I am going along the lines of these experiments I have mentioned. I wonder if you could advise based on this?

That doesn't quite make sense. The bottom sketch shows an electrode stuck into the stem with the other amplifier input connected to ground, but the text says they have a 'reference electrode inserted into the floral foam'. I don't know what floral foam is, but the diagram doesn't show it, it shows ground. I'm guessing they are measuring the potential across the stem, although the picture shows something different.

Floral foam is what they use for fake flower arrangements instead of dirt. I guess it may be a plastic flower there using.

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Or maybe the flower is made of some type of foam material. In either case I guess the material is highly conductive