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

3.3.4 EMI Emissions Testing

The system was tested in a certified EMI test facility according the CISPR-22 emissions standard. During testing, there were two active buttons with a sensor oscillation frequency of 9MHz and an oscillation amplitude of 1.3 VP .

emissions_plot_snoa951.pngFigure 16. EMI data of LDC1612 with CISPR 22 mask

The blue line shows the maximum allowed emissions as per CISPR22 standard. The measured results in Figure 16 show that the LDC solution passes the CISPR22 test across the whole frequency range of interest.

When using short trace lengths between the sensor and the IC, no external components are required to reduce emissions in order to pass this test. When using larger sensors or remote sensing in a system with long wires between the sensor and the IC, external passive components such a common-mode choke and capacitor solution may be needed to pass emissions testing.

figure17.gifFigure 17. Using a Common-Mode Choke to Reduce EMI Emissions