Cypress CapSense CY8C20x36 Betriebsanweisung PDF herunterladen (Seite 33)

Getting Started with CapSense

Document No. 001-64846 Rev. *I 33

3. Design Considerations

When designing capacitive touch sense technology into your application, it is important to remember that the

CapSense device exists within a larger framework. Careful attention to every level of detail from PCB layout to user

interface to end-use operating environment will enable robust and reliable system performance.

3.1 Overlay Selection

In a CapSense design, overlay material is placed over the sensor pad to protect it from the environment and prevent

direct finger contact.

3.1.1 Overlay Material

In the Self-Capacitance Equivalent Model section, Equation 1 shows the finger capacitance.

















Where:

= Free space permittivity

= Dielectric constant of overlay

A = Area of finger and sensor pad overlap

D = Overlay thickness

The geometry of a CapSense system is more complex than a parallel plate capacitor. The conductors in the sensor

include the finger and PCB copper. However, like a parallel plate capacitor, C

is directly proportional to ε

. High

dielectric constants lead to high sensitivity. Because air has the lowest dielectric constant, any air gaps between the

sensor pad and overlay must be eliminated.

Dielectric constants of some common overlay materials are listed in Table 3-1. Materials with dielectrics between 2.0

and 8.0 are well suited to capacitive sensing applications.

Table 3-1. Dielectric Constants of Common Materials

Material



Air

1.0

Formica

4.6–4.9

Glass (Standard)

7.6–8.0

Glass (Ceramic)

6.0

PET Film (Mylar®)

3.2

Polycarbonate (Lexan®)

2.9–3.0

Acrylic (Plexiglass®)

2.8

ABS

2.4–4.1

Wood Table and Desktop

1.2–2.5

Gypsum (Drywall)

2.5–6.0

Conductive material cannot be used as an overlay because it interferes with the electric field pattern. For this reason,

do not use paints that contain metal particles in the overlay.

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