Design Considerations
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X
66
Size of the sensor:
The proximity sensor size depends on various factors, such as the required proximity-sensing
distance, presence of noise sources, and floating or grounded conductive objects. Noise sources and floating or
grounded conductive objects reduce the SNR and the proximity-sensing distance. Therefore, large proximity
sensors are needed to achieve the proximity-sensing distance required in your design.
shows the relationship between the proximity-sensor loop size and the proximity-sensing distance for
a given system. A larger sensor area results in more electric field lines coupling with the target object, resulting in
increase in the sensor signal. However, a large sensor area results in a high sensor C
P
and high noise, and thus
reduces the proximity-sensing distance. Using a loop sensor (
Figure 3-37 (b)
) instead of a solid-fill sensor results
in a low sensor C
P
, low noise, and thus a large proximity-sensing distance. Also, loop sensors require less sensor
area, leaving more space to place components on the PCB.
Figure 3-38.
Proximity Loop Size versus Proximity Distance
Note
In the above graph, proximity-sensing distance for different loop sizes was measured under lab conditions.
The actual proximity-sensing distance varies depending on the end-system environment.
It is very difficult to derive a relationship between the sensor size and the proximity-sensing distance. Depending
on the end-system environment, the proximity-sensing distance may vary for a specific sensor size. You can find
the sensor size needed to achieve a required proximity-sensing distance by making sensor prototypes. You can
use a copper foil, as
shows, to make a quick sensor prototype to determine the sensor size needed to
achieve the required proximity-sensing distance.
Figure 3-39. Proximity Sensor Prototype Using Copper Tape
As a rule of thumb, it is recommended that you start with a minimum loop diameter (in the case of a circular loop) or
diagonal (in the case of a square loop) equal to the required proximity-sensing distance. If you cannot achieve the
required proximity-sensing distance with a loop diameter or diagonal equal to the required proximity-sensing distance,
you can increase the sensor loop diameter or diagonal until the required proximity-sensing distance is achieved.
summarizes the proximity-sensor layout guidelines. If the area available for the proximity sensor is less than
the area required to achieve the required proximity-sensing distance, you can implement firmware filters such as the
Advanced Low Pass (ALP) Filter. The ALP filter attenuates the noise in the sensor raw count and increases the SNR.
An increase in SNR results in a large proximity-sensing distance. Refer to
– Proximity Sensing with CapSense
for details on ALP filter.
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Proximity Loop Size (cm)
Proximity Distance
With ALP Filter
Proximity Distance
Without ALP Filter
