ST AN2967 Application note

AN2967

Application note

Implementing a driven shield

on STM8T and STM8TS capacitive sensors

Introduction

The STM8T and STM8TS capacitive sensors opens up a new dimension to capacitive
sensing by detecting user’s proximity up to 30 cm. This is a key feature for many end­applications such as personal navigation devices (PNDs) or stove tops which are equipped
with backlighting that switches on when a user is detected. Proximity detectors are also
implemented in white goods, automotive devices, palm tops, and on any type of kitchen or
office appliance where the display needs to be turned on to allow some parameters to be
adjusted by the user.

To enhance their application appearance and ease-of-use, designers can choose to add
proximity detection as an extra feature. The alternative is to implement it to save energy,
allowing the product to remain in sleep mode and only wake up when it detects a user in its
proximity.

May 2009 Doc ID 15601 Rev 1 1/9

www.st.com

Interference with proximity detection range AN2967

PCB

Trace between sense pad

Sense pad

Detection distance

1 Interference with proximity detection range

Unfortunately in many cases the end-application consists of a metal enclosure or large
grounded metal objects, as can be seen in stove tops or microwave ovens. Metal objects,
ground planes or traces close by a proximity sensing electrode dramatically influence the
sensor detection range. Refer to Figure 1 and Figure 2, for a comparison of the propagation
of the electrical field without and with metal objects in the electrode proximity.

Figure 1. Electrical propagation from a sense pad and its trace

2/9 Doc ID 15601 Rev 1

AN2967 Interference with proximity detection range

PCB

New detection distance

Ground casing /
metal object

Sense pad

Trace between IC
and sense pad

Figure 2. Electrical propagation from a sense pad and its trace in the presence of a

ground casing / metal object

The propagation from a certain area of a sense pad is directly equivalent to the surface area
of that part of the sense pad. For example, the longer the trace (more extended area)
between the IC and the sense pad, the more the propagation from that trace will be. A
similar phenomenon occurs for the sense antenna.

Figure 1 illustrates the propagation from a trace and sense pad. Adding a ground plane

around the sense antenna causes the electrical propagation to mainly go towards this plane,
as shown in Figure 2. The reduction of sensitivity shown in Figure 2 can also be caused by
components, metal objects or other traces that could results in stray capacitances.

Doc ID 15601 Rev 1 3/9