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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.
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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.
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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 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.
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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).
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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...
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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. ●...
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.
Revision history AN2967 Revision history Table 1. Document revision history Date Revision Changes 04-May-2009 Initial release. Doc ID 15601 Rev 1...
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