AN4312 Datasheet, PDF (7/36 Pages) STMicroelectronics – Guidelines for designing touch sensing applications with surface sensors

AN4312
Capacitive sensing technology in ST
Background
Figure 1. Equivalent touch sensing capacitances
Electrode
Application ^CH
V_SS
C_X
C_T
C_F
Application ground
Earth
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C_X is the parasitic capacitance of the electrode.
C_X is composed of two capacitances: the first one refers to earth, which is not significant
and can be ignored, and the second one refers to the application ground, which is
dependent on the PCB or the board layout. This latter parasitic capacitance includes the
GPIO pad capacitance and the coupling between the electrode tracks and the application
ground. The PCB and board layout must be designed to minimize this parasitic capacitance.
C_F is the feedback capacitance between earth and the application. Its influence is important
in surface capacitance touch sensing applications, especially for applications which do not
feature a direct connection to earth.
C_T is the capacitance created by a finger touch and it is the source of the useful signal. Its
reference is earth and not the application ground.
The total capacitance measured is a combination of C_X, C_F and C_T where only C_T is
meaningful for the application. So we measure C_X plus C_T in parallel with C_F, which is given
by the formula:
Equation 1
C_X + -_C-----1----_T------+¹------_C----₁----_F---
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