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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
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Summary of Contents for ST STM8T
- Page 1 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.
- Page 2 Interference with proximity detection range AN2967 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.
- Page 3 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 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.
- Page 4 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).
- Page 5 AN2967 Possible shield solutions Figure 4. 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 Ground casing / metal object Trace inside shield Doc ID 15601 Rev 1...
- Page 6 Advantages of the shield function AN2967 Advantages of the shield function Some STM8T and STM8TS capacitive sensors have a built-in shielding function. The advantages of the shield include: ● The sensing antenna (pad) is separated from the sealed electronics. ●...
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Page 7: Connecting The Shield
AN2967 Connecting the shield Connecting the shield Ideally, a coaxial cable should be used for the shield. A R (typically 2 kΩ) resistor should be connected to the CX pin corresponding to the sensing electrode to be shielded. The other side of the R resistor is connected to the center core of the coaxial cable. -
Page 8: Revision History
Revision history AN2967 Revision history Table 1. Document revision history Date Revision Changes 04-May-2009 Initial release. Doc ID 15601 Rev 1... - Page 9 No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such third party products or services or any intellectual property contained therein.
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