AN2869 datasheet(6/43 Pages) STMICROELECTRONICS

6 / 43 page

Capacitive sensing technology in ST

AN2869

6/43

Doc ID 15298 Rev 6

1.5

Surface capacitance

A capacitance is modified when a finger gets close to a sensing electrode.

The return path goes either through:

●

a capacitor to ground through the user’s feet

●

a capacitor between the user’s hand and the device

●

a capacitor between the user’s body and the application board through the air (like an

antenna)

Background

Figure 1.

Equivalent touch sensing capacitances

CX is the parasitic capacitance of the electrode.

CX 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.

CF 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.

CT 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 CX, CF and CT where only CT is

meaningful for the application. So we measure CX plus CT in parallel with CF, which is given

by the formula: CX + 1 / ((1 / CT) + (1 / CF)).

Application

Electrode

Earth

Application ground

ai15083

VSS

AN2869 Datasheet(PDF) 6 Page - STMicroelectronics