AN4312 datasheet(27/36 Pages) STMICROELECTRONICS

Part #	AN4312

Description	Guidelines for designing touch sensing applications with surface sensors

PDF	36 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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27 / 36 page

DocID024815 Rev 1

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AN4312

Surface sensor design

If the design is done by using the charge transfer acquisition principle to have an efficient

shield, its waveform must be similar to that of the touchkey. Here are some guidelines for

achieving this (refer to Figure 23 and Figure 24):

•

The Cs/Cx of the shield should be in the same range as the Cs/Cx of the touchkeys.

•

Using Csshield = k.Cskey (c) usually gives good results.

•

The Cs of the shield does not need to be a high grade capacitor. Any type should work.

•

The noise/ESD protection resistor may be mandatory on the shield because it may be

exposed to ESD. In order not to modify the pulse timings, the Rsshield should be in the

range of Rskey/k.

Figure 23. STM8L1xx driven shield example using the charge-transfer acquisition

principle

c. k = (shield area)/(electrode area)

MS18978V3

IO4

IO1

IO2

IO3

IO4

IO3

IO2

IO1

Group 1

Group2

(Shield

Group)

STM8L101

Rskey

Cskey

Rsshield

Csshield

AN4312 Datasheet(PDF) 27 Page - STMicroelectronics