AN1250 datasheet(14/22 Pages) MICROCHIP

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AN1250

DS01250A-page 14

APPLICATION-SPECIFIC

CONSIDERATIONS

Every application will have unique needs relative to its

capacitive touch features. Some of the factors include:

environment, response time, number and type of

capacitive touch sensors, and the number of other

tasks that the microcontroller is ultimately responsible

for (such as USB, graphics, I2C™, SPI, UART, etc).

There are typically three common variables that affect

overall performance of capacitive touch sensors.

These are:

• Printed Circuit (PC) board layout including sensor

pad size, shape and proximity to other sensors

• Material and thickness of covering over the

capacitive touch sensors

• Noise susceptibility, both conducted and radiated

PC Board Layout

Basic considerations for PC Board (PCB) layout

include keeping traces as short as possible. Traces to

capacitive touch sensors should not run for long

lengths in parallel due to capacitive coupling effects.

The PICDEM Touch Sense 2 Demo Board was laid out

so that the most sensitive area (i.e., the matrix keypad)

was closest to the microprocessor, so that the traces

run to the capacitive sensors were the shortest, and

capacitance due to trace length was kept to a minimum.

The subject of board layout practice can be quite intri-

cate; refer to AN1102, “Layout and Physical Guidelines

for Capacitive Sensing

” for more information. The pad

shape, size and proximity to other sensors is also

covered in AN1102. The PICDEM Touch Sense 2

Demo Board uses a half inch (½") diameter pad with a

quarter inch (¼") spacing between adjacent sensors.

The matrix keypad uses half circle sensor pads that

have a ½" diameter. The directional wheel sensors

have a total area that is approximately double that of

the ½" diameter pads. This was required to allow the

double and quad button areas.

Capacitive Touch Covering (Overlay)

Considerations

The material and thickness of the covering over the

capacitive touch sensors is very significant. It deter-

mines how much of the capacitance of a human finger is

actually transmitted to the capacitive touch sensor.

Therefore, thinner is better than thicker. If a material is

too thin, it may not be mechanically sufficient to support

and protect the circuit board. The dielectric constant of

the material also is a determining factor. A higher

dielectric constant will give better results. Two common

materials used are PLEXIGLAS™ and tempered

window glass. PLEXIGLAS has a dielectric constant of

2.5-3.5, while window glass has a dielectric constant in

the range of 4-8. More information on the topic of

covering capacitive touch sensors can be found in

AN1102,

“Layout and Physical Guidelines for Capacitive

Sensing

”.

Noise Susceptibility

The existing noise, which affects capacitive touch appli-

cations, should always be considered. The factors

affecting how to protect against noise are very applica-

tion-specific. The PICDEM Touch Sense 2 Demo Board,

for example, is open on both the front and back. A

hatched ground pattern was added to the back of the

board wherever possible to minimize the effects of noise

that could be introduced from the bottom of the board.

The cross hatched pattern provides a good path to

conduct unwanted noise to ground, and at the same

time, it does not cause an appreciable increase in the

capacitance of the touch sensors due to the coupling

effect of the sensor pad through the PC board material

to the ground plane.

Some applications may have a completely shielded

enclosure which would allow for less ground plane

added to the PC board. Another consideration is the

proximity of the capacitive touch sensors to other noise

producing circuitry on the board, such as communication

busses, inverting power supplies for back lighting dis-

plays, RF radio transmitters such as those used for Wi-Fi

or other radio based communications, and so on. In the

case of a radio transmitter and capacitive touch sensors,

it is best to read the buttons only when the RF transmitter

is disabled.

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