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 end-
applications 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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Doc ID 15601 Rev 1
AN2967
Figure 2.
Interference with proximity detection range
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
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Possible shield solutions
AN2967
2
Possible shield solutions
The driven shield available on selected STM8T/STM8TS capacitive sensors can be used to
decrease the negative influence a ground plane or metal object on the proximity detection
range. The shield should always be at the same potential as the sense antenna.
For this reason some STM8T/STM8TS devices feature a SHLDIN pin to which a CX channel
can be connected (see
Figure 5).
The SHDLOUT pin then has the same potential as the CX
channel.
The shield can be implemented in two different ways:
●
●
The shield can protect a whole system as shown in
Figure 3.
The shield can protect only the trace. In this case the sense antenna is isolated from
any interference source (i.e. ground plane, metal object, other traces, user influence on
that trace) as shown in
Figure 4.
Shield implemented as pour between sense pad and ground casing /
metal object
Figure 3.
Detection distance
Shield implemented as
pour on bottom of PCB
Sense pad
PCB
Ground casing /
metal object
Trace between IC
and sense pad
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Doc ID 15601 Rev 1
AN2967
Figure 4.
Possible shield solutions
Shield implemented as coaxial cable shielding trace between sense pad
and IC from ground casing / metal object
Detection distance
Shield
(implemented as coaxial cable)
Sense pad
Trace inside shield
Ground casing /
metal object
Doc ID 15601 Rev 1
5/9
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