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

2.2.4 Other Considerations

In addition to the design considerations that are discussed in this document, other factors must also be taken into account when designing a liquid-tolerant capacitive touch system.

For example, some applications require the capacitive touch system to work properly with liquid present while the user wears gloves to touch the panel. In these cases, the designer must define the expected use conditions and also to consider the tradeoff between responsiveness to gloved fingers and liquid-tolerant performance. The material, structure, and thickness of the glove can significantly affect capacitive touch performance parameters (such as sensitivity). The different capacitive sensing methods (self and mutual) also behave slightly differently when gloves are used while touching the sensors.

Other common design considerations include:

  • Mechanical stackup, which can affect touch sensitivity and reliability
  • Environmental temperature drift, which can cause a drift in the sensing measurement result
  • Subsystem interconnection, which can affect the touch sensitivity and introduce additional noise to the measurement result

Overall, it is critical for a product designer to consider typical use cases and system factors (such as the ones listed above) that can affect the capacitive touch performance. This document does not describe all of these factors. This reference design is configured to reliably detect bare-finger touch when exposed to a continuous stream of water and a 3-Vrms conductive noise coupling directly to the power supply while eliminating false touch detections.