TIDUE90 July   2018

 

  1.    Description
  2.    Resources
  3.    Features
  4.    Applications
  5.    Design Images
  6. 1System Description
    1. 1.1 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Classification of Scenarios With Liquid Present
      2. 2.2.2 Liquid Influence on Capacitive Touch Sensing
      3. 2.2.3 Self Capacitance and Mutual Capacitance
        1. 2.2.3.1 Self Capacitance
        2. 2.2.3.2 Mutual Capacitance
      4. 2.2.4 Other Considerations
    3. 2.3 Highlighted Products
      1. 2.3.1 MSP430FR2633
    4. 2.4 System Design Theory
      1. 2.4.1 Shield Sensor Electrodes
      2. 2.4.2 Mutual Capacitance Shielding
      3. 2.4.3 Design for Noise Immunity
      4. 2.4.4 Power Supply Grounding Effect
  8. 3Hardware, Software, Test Requirements, and Test Results
    1. 3.1 Required Hardware and Software
      1. 3.1.1 Hardware
      2. 3.1.2 Software
    2. 3.2 Test and Results
      1. 3.2.1 Liquid Test With Well Grounded Power Supply
        1. 3.2.1.1 Continuous Water Flow Test
        2. 3.2.1.2 Continuous Water Spray Test
      2. 3.2.2 Conductive Noise Immunity Test
      3. 3.2.3 Liquid Test With Battery-Powered Supply
        1. 3.2.3.1 Continuous Water Flow Test
        2. 3.2.3.2 Continuous Water Spray Test
      4. 3.2.4 Third Party Test Report
  9. 4Design Files
    1. 4.1 Schematics
    2. 4.2 Bill of Materials
    3. 4.3 PCB Layout Recommendations
      1. 4.3.1 Layout Prints
    4. 4.4 Altium Project
    5. 4.5 Gerber Files
    6. 4.6 Assembly Drawings
  10. 5Software Files
  11. 6Related Documentation
    1. 6.1 Trademarks
  12. 7About the Author

2.4.2 Mutual Capacitance Shielding

A mutual capacitance sensor requires a RX electrode and a TX electrode. Typically, mutual capacitive electrodes can be multiplexed with other mutual capacitive electrodes. This means that more than one button can share a common signal. However, to optimize the sensor design for liquid tolerance, the design needs a driven shield to protect the sensors from coupling to surrounding ground or other components. To accurately detect the touched sensor during heavy continuous liquid flow, the design should have an individual RX signal for each sensor instead of multiplexing the RX signal with other sensors.

Unlike in the self capacitance electrode design, in which the hatched filling connects to a CapTIvate I/O for the driven shield, in mutual capacitance mode, the hatched filling connects to the TX signal. This hatched filling is used as a driven shield to protect all of the sensors and the shared TX electrode for all the sensors. For each individual RX electrodes, use a solid pad instead of hollow traces to protect the sensors from coupling to the ground or other components on other layers of the PCB. All of these recommendations about the design of mutual capacitance sensors are optimized for a keypad application that can operate reliably when exposed to heavy continuous liquid flow.

Figure 10 shows the layout of the 12-button keypad. For more details on the layout design, refer to Section 4.3 or download the design files from the TIDM-1021 page.

TIDM-1021 tida-1021-drawing-of-the-12-buttons-keypad-mutual-capacitance-design-layout-block-diagram.gifFigure 10. 12-Button Keypad Mutual Capacitance Design Layout