SNOA951 June   2016 LDC1312 , LDC1312-Q1 , LDC1314 , LDC1314-Q1 , LDC1612 , LDC1612-Q1 , LDC1614 , LDC1614-Q1

 

  1.   Inductive Sensing Touch-On-Metal Buttons Design Guide
    1.     Trademarks
    2. 1 ToM Basics
    3. 2 How Are Inductive Touch-On-Metal Buttons Implemented?
    4. 3 System Design Procedure
      1. 3.1 Mechanical System Design
        1. 3.1.1 Designing for Natural Button Force
          1. 3.1.1.1 Metal Composition
          2. 3.1.1.2 Metal Thickness
          3. 3.1.1.3 Mechanical Structure of the Button
        2. 3.1.2 Target Distance
        3. 3.1.3 Mechanical Isolation
        4. 3.1.4 Mounting Techniques
      2. 3.2 Sensor Design
        1. 3.2.1 PCB Design
        2. 3.2.2 Sensor Frequency Selection
        3. 3.2.3 Sensor Amplitude Selection
      3. 3.3 Other Considerations
        1. 3.3.1 Button Quantity and Multiplexing
        2. 3.3.2 Power Consumption
        3. 3.3.3 Software Algorithm
        4. 3.3.4 EMI Emissions Testing
      4. 3.4 Design Implementation
    5. 4 Results
    6. 5 Summary
    7. 6 Additional resources

2 How Are Inductive Touch-On-Metal Buttons Implemented?

Inductive-to-Digital Converters (LDC) are able to measure proximity to metal by detecting the subtle changes in an AC magnetic field resulting from the interaction with the metal target. The LDC generates an AC magnetic field by supplying an AC current into the parallel LC resonant circuit shown in Figure 2.

figure2.gifFigure 2. LC Sensor Components

If a conductive target is brought into the vicinity of the inductor’s AC magnetic field, small circulating currents known as eddy currents will be induced by the magnetic field onto the surface of the conductor shown below in Figure 3.

figure3.gifFigure 3. Eddy Currents Induced on Metal Surface

These eddy currents produce their own magnetic field that opposes the one created by the inductor which reduces the effective inductance of the coil. The resulting inductance shift is measured by the LDC and can be used to provide information about the position of the target over a sensor coil such as distance or equivalently the force of a button press.