RC Acquisition Principle

The RC acquisition method is used to detect the human touch of any capacitive touch by measuring the small variation of the touch electrode capacitance. Electrode capacitance (C) is periodically charged and discharged through a fixed register (R). The capacitance value (C) is calculated by measuring the charge time (t1) the (Vout) voltage requires to reach the threshold (Vth).

In touch sensing applications, the capacitance value (C) is the addition of a fixed capacitance (electrode capacitance, Cx) and the capacitance added by the human finger (touch capacitance, Ct) when it touches or is close to the electrode. Correspondingly, it takes more time (t2) to reach the threshold (Vth) during charge and discharge. The increasing from (t1) to (t2) indicates that a finger has touched the electrode capacitance.

Reference:

• http://www.st.com/stonline/products/literature/an/15323.pdf