AD7747 - C2D converter - Issue

Hi everybody, 

I want to do a level sensor for my own by using: 

- copper strips on a thin plastic glass

- AD7747 

- Arduino UNO

The issue is that I am not really familiar with I²C communication since it is the first time I work with that. Therefore, by finding documentations on internet and in the datasheet, I wrote a code to communicate with my AD7747. The code is actually working but it always displays 8.20...pF and it seems that whatever  I do, the measured capacitance does not change at all. 

About the application of my level sensor, when I measured manually the capacitance between the two plates, it varies from 5pF (Low level) to 35pF (High level), the level I want to reach is situated around 20pF so I was wondering how to "calibrate" the AD7747 so I can obtain this kind of measurement from my chip: 20pF +- 8pF ?

- 20pF is my wished level and the variation (+-8pF) is the full range of the chip.

To help you understanding what I did, please find in attachment a wiring diagram of my sensor and the code I wrote. About the picture, I know it is written AD7745 on it but I currently use the AD7747, the latter was not available in the library of the program I used.

I hope that someone can answer me or provide me a code that I can use to finish my level sensor. 

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