PCB
Standoff
Standoff
Button Surface
Metal Target
(0.2mm)
1mm
1mm
Force
Figure 2-3. Inductive Touch Button Stackup
The material of the button surface has an impact on how much deflection will occur to the metal target. Materials
that are more stiff or that absorb the force will cause less deflection and therefore will require more force for a
button press to be detected. This also comes into play when considering the thickness of the button surface. The
LDC Calculator Tool Spreadsheet
has a tab for determining the deflection of a material if the Young's Modulus
and Poisson Ratio are known for the material. Since this design is 3D printed using nylon 12, a deflection of
around 20 μm is expected for a 2N force applied to the surface. This amount of deflection will be plenty for this
button design since the target is so close to the sensor to begin with.
Using metal tape or a small metal target on the inside of the button surface allows for non-metal materials to
work in a button fabrication. Performance will vary depending on the metal used for the target. It is desired to
use a metal with a high conductivity to maximize the sensitivity of the button. Because of this, both copper and
aluminum tape are strong options since they have a high conductivity and can easily be cut to fit within the
spacers for the button design. An alternative approach to this is having a metal layer attach to the button surface
and placing the spacers between the metal layer and PCB sensors such as in the
Touch System Design Guide for HMI Button Applications
application note for more information on button design.
PCB
Spacer
Spacer
Non-Conductive Rigid Surface
Conductive Target
Force
Stiffener (Optional)
Figure 2-4. Non-Conductive Touch Button Alternative Stackup Example
2.2.2 Sensor Coil Placement
The LDC3114 allows for sensor coils to be placed remotely rather than next to the IC due to the addition of the
COM pin and placing the sensor capacitor near the device instead of next to the coil. This provides a better EMI
response for the coil and eliminates the need for additional filtering in most cases. Due to the sensor coils being
placed at different distances from the IC, their trace lengths vary and cause a slightly different series resistance
value for each coil. This can also lead to a slightly different resonant frequency of the different sensors but since
the button application uses the baseline tracking algorithm, it does not need to have an exact measurement on
each channel and cares more about the change in the data than what the data value starts at.
2.2.3 Collecting Data from Multiple LDCs
Since the LDC3114 uses the same I2C address, an I2C mux is used to communicate with each device
independently. Each LDC3114 is driving four sensor coils at the default 40SPS. Since touch buttons do not
require high-speed operation, the sample rate does not need to be increased and polling each device for their
data does not cause a latency issue on the touch button. For battery powered applications, the sample rate can
be decreased and the digital outputs of the LDC3114 can be monitored instead of using the I2C data. In this
design, there were limited GPIO ports on the connector to the main controller so the digital outputs are read from
the OUT register of each device instead.
System Overview
6
Inductive Touch and Magnetic Dial Contactless User Interface Reference
Design
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