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AD5933 Feedback and Calibration Resistance

Question asked by richeekdey on Dec 17, 2017
Latest reply on Dec 18, 2017 by richeekdey

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. 

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