I am using the AD7746 to measure a capacitance change when a floating (non-grounded) chip moves over the drive (EXCA) and receive (CIN+/-) electrodes that make up the sensor. I understand the AD7746 uses mutual capacitance to make these measurements, where an object interacts with the fringing field of the sensor effectively reducing the capacitance seen by the sensor (i.e. http://atmelcorporation.files.wordpress.com/2012/12/fig1_pg21.jpg).
However, during my tests the chip always increases the capacitance output from the AD7746. In order to optimize the geometry for the application, I would like to understand why im getting results that are counter intuitive to the normal mutual capacitance operation. Is it because the chip is floating and coupling with the electrode? Is there some other complex interaction going on? These tests are being done at the micro scale if that has any effect.