Layout and Physical Design Guidelines for Capacitive Sensing

AN1102 Layout and Physical Design Guidelines for Capacitive Sensing FIGURE 1: EXAMPLE PAD SIZES AND Author: Tom Perme SHAPE Microchip Technology Inc. 0.500 x 0.500 INTRODUCTION (12,7 x 12,7) This application note describes the layout and physical design guidelines used for the capacitive sensing solu- 0.188 x 0.188 tion proposed in AN1101 "Introduction to Capacitive (4,7 x 4,7) Sensing". The layout and physical design of your capacitive system is an important part of the design process. A good layout will make the software imple- Again, the shape is not the key parameter; a circle of mentation simpler. Depending on the application, the approximately the same area will function comparably layout may be very simple, or more complex, but the to the square shape suggested. same simple guidelines govern all layouts. Sometimes a button is shaped for aesthetic purposes. A simple way exists to make a very nice looking inter- PAD SHAPE AND SIZE face to a person. By putting a printed paper with graphic designs between the pad and a clear touch sur- General Guidelines face, the user will see the graphic paper while the actual pad is hidden below. The paper may have the When designing a capacitive button, the shape of the complex shape on it, meanwhile below the paper, a pad is not very important. The area of the pad is the simple, less artistically demanding copper pad can parameter to design for. A larger pad area will allow exist with a simple shape. An example is shown in better detection and sensitivity. A smaller pad has Figure 5. poorer detection capability. Also, a greater distance, between capacitor plates reduces capacitance as in EFFECTS OF COVERING PLATEEquation 1. As a rule of thumb, the area should be about the size of an average person's finger when ® Window glass and Plexiglas are common materials pressed against the button; for example, a square for use as the surface which a person touches. These 0.5" x 0.5" (12,7 mm x 12,7 mm) makes a good sensor. common materials come in various thicknesses, and This very simple shape is easy to design and easy to the thickness and composition of the material between implement in a grid of buttons. the pad and touching surface affects sensitivity. When comparing window glass to Plexiglas, or another brand EQUATION 1: CAPACITANCE EQUATION acrylic, the window glass will allow detection through a thicker piece of material given identical testing condi- ? ? A o r C = tions. This is because the dielectric constant of window d glass is higher than the dielectric of acrylics. Numerous specifications for a particular acrylic or type of glass Another related concern is the proximity of a button to exist, but the dielectric constants are on the order of 2- adjacent buttons. When a person touches a sensor, or 3 for acrylics and about 7 for window glasses. Other its covering plate (plastic, glass, etc.), the person's fin- notable substances have dielectric constants of 1 for air ger