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

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

Detection distance

PCB

Sense pad

Trace between sense pad

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AN2967	Interference with proximity detection range

Figure 2. Electrical propagation from a sense pad and its trace in the presence of a ground casing / metal object

New detection distance

Sense pad

Trace between IC

and sense pad

PCB

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