Hi all,
As I have also go over the document 'Ground Shifting in Capacitive Sensing Applications' for FDC2X1X, the grounding issue still confused me a lot.
For human proximity, the EVM chip itself is Floating Ground Referenced to battery (Application ground). And the capacitor sensor is also Floating Ground Referenced to human body (virtual ground). So there should be feedback capacitance between earth and the application (CF) and capacitance created between the virtual and real earth ground (CT), right?
If there is the such two capacitors, than the total capacitance measured is a combination of proximity capacitance(Cp), CF and CT where only Cp is meaningful for the application. So we are actually measuring CX in series with CT and CF which is given as C(total) = 1 / ((1 / CX) + (1 / CT) + (1 / CF)). C=(ε0×εr×A)/d and all the capacitor have the same dielectric constant for air, so the contribution of each capacitor would determined by d/A ratio.Compared with CX, CT has similar level of area but much larger distance. Why it doesn't dominant the final measurement result.
Wenbing
