Design Guide SLAA576AMay 2013Revised November 2015 Capacitive Touch Hardware Design Guide Holly Gu, Chris Sterzik.MSP430 ABSTRACT MSP MCUs with CapTIvate technology are the most noise-immune capacitive-touch MCUs and first to feature an IEC61000-4-6 certified solution. The MCUs feature the most configurable combination of capacitive buttons, sliders, wheels, and proximity sensors, all at the worlds lowest power. To learn about these new solutions, go to Capacitive touch detection is sometimes considered more art than science. This often results in multiple design iterations before the optimum performance is achieved. There are, however, good design practices for circuit layout and principles of materials that need to be understood to keep the number of iterations to a minimum. This design guide describes a process for creating and designing capacitive touch solutions, starting with the schematic, working through the mechanicals, and finally designing the electrodes for the application. Contents 1Introduction.2 2Overview of Capacitive Touch Sensing.2 3Schematic.6 4Mechanicals7 5Layout11 6References22 List of Figures 1Sample Electrode Shapes3 2Equivalent Circuit4 3Typical Material Stackup8 4Sensitivity vs Thickness.9 5Intentional Gaps Between Electrode and Overlay.11 6Top and Cross-Sectional Views of Trace Line in PCB12 7Trace Without Copper Pouring Underneath in Multilayer PCB.13 8Top View of Connector and Noise Source14 9Top View of Different Traces.15 10Capacitance of the Electrode.16 11Button Shape Examples, Dos and Donts.17 12Incorrect Design of a Slider or Wheel for Desired Resolution18 13Designing Electrode Shapes to Support Desired Resolution.19 14Ends of Sliders19 15Effective Area Example for Electrodes Larger Than Decal.20 16Effective Area Example for Electrode Smaller Than Decal20 17Ground Separation.22 18Example Hatched Ground Fill22 CapTIvate, MSP430 are trademarks of Texas Instruments. 3M is a trademark of 3M. Gorilla is a registered trademark of Corning Incorporated. 1SLAA576AMay 2013Revised November 2015Capacitive Touch Hardware Design Guide Submit Documentation Feedback Copyright 20132015, Texas Instruments Incorporated I 1Introduction Texas Instruments provides three major pieces of collateral for developing capacitive touch solutions: 1. This document, the Capacitive Touch Hardware Design Guide 2. Capacitive Touch Software Library (CAPSENSELIBRARY) 3. Tuning guides: Capacitive Touch Sensing, MSP430 Button Gate Time Optimization and Tuning Guide (SLAA574) Capacitive Touch Sensing, MSP430 Slider and Wheel Tuning Guide (SLAA575) Good sensor design is the foundation for a successful touch product. The purpose of this design guide is to provide guidance for the design and layout of capacitive touch sensors so that they can achieve maximum performance. By achieving maximum performance in the hardware, the capacitive touch software library can perform the capacitive touch measurements with the lowest power consumption. After the software library is implemented for the application, the tuning guides can be used to tune the performance of the capacitive touch application. Section 2 provides an overview of capacitive touch detection and describes its terms, concepts, and theory of operation. Section 3 and following are organized like a product designfrom schematic to mechanicals to layout. The schematic and mechanicals are treated as design requirements that influence the PCB layout. This guide first describes requirements that might be found in the schematic and mechanical requirements of the product. These requirements have an effect on the layout and must be considered first. After the requirements of the product are understood, then the PCB layout can begin. 2Overview of Capacitive Touch Sensing This overview is broken into two main parts. The first part is the theory of capacitive touch detection. In Section 2.1, some basic terms are defined and discussed to give a basis for the description of capacitive touch detection provided in the equivalent circuit. The second portion of the overview in Section 2.2 introduces figures of merit used in capacitive touch detection. These parameters are used to evaluate the performance and are referred to frequently in the document as the goal or reason for certain design rules. 2.1Theory of Capacitive Touch Sensing Figure 2 shows an equivalent circuit that provides a basis for understanding how the different aspects of the design contribute to the overall performance. Before discussing the equivalent circuit, some basic terms are introduced: electrode, traces, and capacitance. 2.1.1Terms: Electrodes, Traces, Capacitance An electrode is the physical conductive structure that a person interacts with. This structure is typically thought of as the copper on a printed circuit board (PCB), but can also be made of transparent materials such as indium tin oxide (ITO) or other conductive materials like silver. As shown in Figure 1, the electrode shape c