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Hardware
The sensitivity of a grounded system is more significant compared to a floating system, especially if the
intended target is the human body. The human body will have a voltage potential close to earth ground
(same or close to system ground) while the voltage potential of the floating system can be significantly
different. This difference in potential directly corresponds to a difference in sensitivity or sensing distance.
This EVM uses a AA-battery as its power source so
2.2
Proximity Sensor
The proximity sensor on the board was designed to detect a human hand at 50 cm using a grounded
system and 10 cm with a floating system. Sensitivity is defined as maximum sensing distance, at which
point the shift in the measured capacitance exceeds the detection threshold, which is determined by the
system noise level. Figure 4 shows a plot of the sensing distance the FDC2214 can achieve with a given
sensor size area in a grounded system. A solid square sensor was used to collect the data in
The proximity sensor on the EVM is designed around the edges of the board in a bezel configuration. The
actual sensor size area is 59 cm
are located in the middle of the proximity sensor.
One of the major factors that affect sensitivity of a proximity sensor is the size and location of the nearest
ground potential. The layout of the board minimizes the ground plane as much as possible. A ground
plane underneath the sensor significantly reduces sensitivity. For example, if a ground shield is directly
below and the same size as the sensor, sensitivity is reduced by 50%. Noise in the system and in the
environment also affects sensing performance.
The system was optimized and tested in a low noise environment. Variations and false triggers may occur
depending on the surrounding environment. The algorithm thresholds in the firmware may need to be
adjusted accordingly to fit a specific environment condition
2.3
Capacitive Touch Buttons
The circular buttons measure 20.32 mm (0.8 inches) in diameter. Capacitive touch buttons require less
attention to sensitivity in terms detecting small changes in capacitance simply because the intent is to
detect a touch event. Typically, changes in capacitance on the order of picofarads are required in
detecting button presses. The same factors that affect sensitivity for proximity still apply for capacitive
touch buttons. A solid or hatched ground pad/shield underneath the button sensor can be used to
moderate the sensitivity of the button and mitigate unintentional touch events.
In most applications, the user interacts with the button sensor directly. In some applications, there may be
a protective cover or a piece of isolation material to prevent direct contact with the sensor (for quality and
reliability purposes). Sensitivity performance is dependent on the material and thickness of the material.
Figure 5
shows a simulated sensitivity analysis for a proximity sensor. Capacitive touch buttons rely on a
large response to determine when a press is detected directly on the sensor. The simulation shows that at
close range, including the touch condition, a protective cover (in this case PVC) can actually enhance the
response. However, this effect is observed over a limited range and at longer ranges shows little impact.
4
FDC2214 Proximity and Capacitive Touch EVM User's Guide
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Sensor Area - square sensor (cm
Figure 4. Sensor Size Area Versus Sensing Distance
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with the spatial sensor size area spanning 121 cm
Copyright © 2015, Texas Instruments Incorporated
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D002
www.ti.com
Figure
4.
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. All of the electronics
SNOU139 – October 2015
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