Figure 2-2. TIDA-060039 LDC Sensor Coil Layout
Based on the coil diameter, the target distance is determined to be 0.8 mm away from the coil. This puts the
target well within the recommended 3%-20% range of the coil diameter which will give a high sensitivity in
response to a force on the button surface. Going back to the calculator spreadsheet, the target distance can be
input to double check the design choices. With the target at 0.8 mm, the sensor frequency is now 11.081 MHz
with a Q factor 0f 26. The spreadsheet will give a warning if any of the final parameters are out of range for the
device, but in this case, no warnings appear.
Table 2-1. LDC Coil Calculator Outputs
Description
Designator
Value
Unit
Total Inductance with no target
L
TOTAL
1.604
µH
Sensor Operating Frequency no target
f
RES
8.396
MHz
R
P
with no Target
R
P
3.19
kΩ
Q factor
Q
37.00
Self resonant frequency (estimated)
SRF
62.831
MHz
Target Distance
D
0.800
mm
Sensor Inductance from Target Interaction
L'
0.921
µH
Sensor Frequency with Target Interaction
f
RES
'
11.081
MHz
R
P
with Target Interaction
R
P
'
1.68
kΩ
Q Factor with target
Q'
26.0
Since the button surface is 3D printed, it also includes the required spacer for the button design instead of
having a separate spacer material. The standoff and button surfaces are both 1 mm thick to provide a surface
that has some slight flexibility and to set our target height as desired. The metal tape is then placed inside the
button surface between the standoffs so that it deflects in the desired area when a force is applied to the button
surface. The thickness of the tape then puts the target at about 0.8 mm away from the sensor coil.
System Overview
Inductive Touch and Magnetic Dial Contactless User Interface Reference
Design
5
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