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AN3150 Datasheet(PDF) 10 Page - STMicroelectronics |
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AN3150 Datasheet(HTML) 10 Page - STMicroelectronics |
10 / 25 page Capacitive system ground AN3150 10/25 DocID17048 Rev 3 Figure 4. Equivalent connected to earth application schematic CLE1 and CLE2 are the capacitances between the power lines and earth. They are both intrinsic to the power network. They usually have relatively high values and are seen as a short from a touch sense capacitance measurement. CLG1 and CLG2 are the capacitances between the power lines and the application ground. The values of these capacitances depend on the power supply design of the application. For example, they may include the capacitance between the primary and secondary coils of a transformer. In most cases, CLE and CLG are high enough to be seen as a short by the touch sensing system. However, in some specific designs it is possible to improve the sensitivity by adding a physical capacitor across the power supply. On plugged devices, CLE1 and CLE2 capacitances in series with CLG1 and CLG2 capacitances, are the main components of the feedback capacitance CF. 3.2 Portable applications In portable configuration, It helps to consider the user’s potential as a reference point instead of the earth potential. In this case, CF is the capacitance between the user’s body and the application ground, see Figure 5: Equivalent portable application schematic. The user’s potential includes some stray capacitances like the one between the user’s hand and the battery. If a portable system is not moved, CF stays relatively fixed. Therefore, in normal operation, while a user approaches the electrode, CT changes and is detected as an activation. Note that the further the module is removed from earth, the weaker the coupling between the module and earth (yielding a smaller CF capacitance). Because CT and CF are in series, a small CF decreases the effect of CT, and the system becomes less sensitive. The variable nature of both parameters CT and CF adds one more complication to the design of portable applications. ai17443V2 Application Electrode CX CT CLE2 CLE1 CLG2 CLG1 |
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