Cypress CapSense CY8C20x36 Guía de usuario descargar pdf (Pagina 38)

38 Document No. 001-64846 Rev. *I Getting Started with CapSense

ground flood is not recommended within 1 cm of CapSense sensors or traces due to an increase in the parasitic

capacitance. Figure 3-6 shows an example of an improper grounding scheme. The layout greatly improves by

reducing the loop area.

Figure 3-6. Improper Ground Scheme and Ground Loop

CapSense

Controller

On Board Driver

circiuts

External Circiuts

1) Decoupling loop inductance and

switching currents combine to create

an AC voltage on the local ground

2) Each output (and input) drives the AC

voltage out onto the PCB. Each signal

will have a loop area associated with it.

3) Cables magnify the problem as loop

areas are proportional to cable length

In Figure 3-7, two sensors are surrounded by a ground plane that is connected to CapSense controller ground, while

a third sensor is surrounded by ground. The third sensor is connected to the other ground plane through the long

traces of other circuitry, which creates a large current loop. With this layout, the third sensor may be more susceptible

to radiated noise and have increased emissions. These two sections of ground are the same location on the

schematic, so they can potentially be one connected area with a better layout.

Figure 3-7. Improper Current Loop Layout

CapSense

Sensor

CapSense

GROUND

FILL #1

GROUND

FILL #2

ISOLATED

OTHER

CIRCUITRY

PATH TO

SENSOR PAD

RETURN

PATH

PCB

Figure 3-8 illustrates the proper layout for the previous example. The loop area is reduced by connecting the two

grounded areas.

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Cypress CapSense CY8C20x36 Guía de usuario Pagina 38