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AD7150: Capacitive effects as conductive and non-conductive materials approach the sensor

If a finger, metal or water or other conducting materials approach the
sensor, the capacitance values are decreasing. If a non-coductive plastic item 
approaches the sensor, first the capacitance values are decreasing but at
closer distances
the values appear to be increasing. So it is difficult to distinguish if a
conducting object is
approaching or a non-conductive object is moving away from the sensor.

 

The same phenomenon happens with both conductive and non-conductive materials.
When a part is initially brought into the electric field, some charge is
diverted to ground and a lower capacitance is measured on the CINx pin. When it
is close enough, the part increases the effective ε and this increases the
capacitance. Conductive materials will divert more charge to ground than
non-conductive materials; therefore the drop in capacitance has a bigger effect
than the increase in ε.
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