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Parallel plate capacitance to co-pplanar capacitance

Question asked by richeekdey on May 23, 2018
Latest reply on Jul 4, 2018 by richeekdey

Probably out of the scope of this forum, but we're all engineers here, so I thought I would get some feedback/inputs. 

So in my previous experiment, I made a parallel plate capacitor by putting 2 copper plates parallel to each other (the third figure in the image). I then hooked up the AD5933 evaluation board (which is an impedance analyzer) and then I put in things in between and observed the dispersion in biological materials (fruits and veggies).

My task now is to "open" up the plates to they're co-planar. I have done that and placed an active shield at the back of the plates to direct my electric fields. However, now I see no observable change on the impedance analyzer when I bring myself or fruits/veggies close to the fields. I do "feel" the field since I sense a bit of warmth closer to the plates. I observed the oscilloscope which is hooked up to the end of the receive plate and I don't see much of an observable difference, rather I see a very small change.

PS...even in my parallel plate configuration, the change in the oscilloscope observed is very minimal.

My question is NOT about why I am NOT getting any change. I know the reason, I would just like to validate it before I take it up with my superiors.

I think the reason that I am NOT observing an change of impedance is because a) the impedance analyzer requires a calibration for the open-plate setup ie: the capacitance across the open plates is obviously NOT the same as the capacitance across the parallel plates. And change of capacitance = change of impedance = new calibration range needed. And b) also, co-planar capacitance is complex and has to be simulated and only then will I arrive at a value.

So would it be right to think that I first need to analytically look at the open-plates and then based on some simulations, apply the formula/value to calibration for the impedance analyzer.

Another fact is that the impedance analyzer cannot measure below 1kOhm. This value can actually be lesser; but in my case I'm using the evaluation board so its limited to 1kOhm - 1MOhm. But this of course comes in WHEN I know the capacitance of these co-planar plates.