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When using capacitive sensing for touch applications the PCB trace typically acts as one plate of the capacitor. When a conductive object, such as a finger for touch sensing, comes into proximity it acts as the other plate and the air is the dielectric. As the object moves closer to the pad the capacitance changes, based on the equation for capacitance shown in this slide. The constants in the equation represent the value for the dielectric constant of the material, i.e. air, glass etc. The value for the term “A” is a measure of the area of the pad used for the sensor capacitor and the term “d” represents the distance between the two plates. Therefore, for touch applications, as the finger moves closer to a pad, the value for “d” is reduced which increases the capacitance.
PTM Published on: 2012-05-16
