# Hi,

So I'm performing a bioimpedance measurement ( I know this has been asked a lot, but mine is different, so please bear with me). My measurement range is 1kHz - 100kHz. I had "made" a capacitor by putting 2 copper plates at a distance of 40mm and then trying to disturb this dielectric and observing the changes. I have attached it below. Now, for air as a dielectric I am getting a value of 3.3pF. Given my characteristics ( d = 40mm and A = 8100 mm2 and k = 1), I expect 1.79pF but this being the ideal case I get 3.3pF. I have cross-checked this with a Solartron impedance analyser which gives 3.2pF. For the measurement of air, I have used RFB = 715K and RCAL = 1.4M. I have used the formula mentioned in the application note AN-1252.

Now, my question is, how do I measure with water as a dielectric. Ideally, with water as dielectric, keeping other params the same, I should get a capacitance of 143pF. However, the Solatron shows a capacitance of 1nF. So now I am a bit confused since I need to know my impedance range to calibrate the device. I have used a water bath with deionised water.

I am interested in the permittivity/dielectric and its change, and not the absolute value of capacitance.

Since I am only interested in the change of capacitance (that is my unknown impedance), would it be possible to replace RFB with a capacitor? If so, what would the formula be? How do they arrive at the formula for RFB? I see in the circuit diagram its just a feedback placed after the unknown impedance. So in principle it should be possible to use a capacitor, right? And would it also be possible to replace RCAL with a capacitor? I know the datasheet mentions "Do Not Calibrate the System with a Complex Impedance", so perhaps RCAL has to remain?

PS.. I am using the EVAL-AD5933EBZ board. So I have the AFE already.

Nope, rule of thumb is that the gap should be at least one order of magnitude narrower than the smallest plate dimension, that would be 9mm in your case.