AN3150 datasheet(6/25 Pages) STMICROELECTRONICS

6 / 25 page

Capacitive sensing overview

AN3150

6/25

DocID17048 Rev 3

Figure 2. Equivalent measured capacitance

We can deduct from Figure 2 the application measures not only CT, but a combination of CX,

CF and CT (see Equation 1).

Equation 1

The challenge in developing a capacitive sensing system is overcoming possible sensing

errors coming from changes in CX and CF that are not created by a touch but, by a change

in the application surroundings.

All the sensors include a means of compensating slow changes in the surroundings but,

they cannot automatically compensate for a fast signal change and report a false spurious

detection if the change is in the same direction as the touch/proximity. For this reason, it is

recommended to recalibrate the device if the application detects changes in the

environment (for example, when the USB cable is plugged/unplugged or the application

door is closed).

Improving the capacitive sensing system sensitivity can be achieved by:

•

Minimizing the CF effect, also known as improving the application grounding.

•

Minimizing the CX value. This value is impacted by the electrode and the overall

application design. It can be improved by using an active shield.

ai17441V2

Application

Electrode

Cmeasured = CX +

Cmeasured

CX 1

() 1CT

⁄

1CF

⁄

()

⁄

AN3150 Datasheet(PDF) 6 Page - STMicroelectronics