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

2.2.3.1 Self Capacitance

The self capacitance method measures changes in capacitance with respect to earth ground. When a user's finger touches the self capacitance sensor electrode, the finger provides a path that couples the sensor to earth ground (see Figure 3). Because of this, an additional touch capacitance is added to Cself, which increases the capacitance. When a liquid covers the touch surface, the liquid also provides a path that couples the sensor to the surrounding ground on the sensor PCB. The additional capacitance Cliquid is added to Cself and increases the capacitance.

With the self capacitance method, both the finger touch and liquid presence increase the sensed capacitance, which means that the self capacitance method is sensitive to liquid on the touch surface.

TIDM-1021 tida-1021-liquid-influence-on-self-capacitance-method-a-block-diagram.gif
TIDM-1021 tida-1021-liquid-influence-on-self-capacitance-method-b-block-diagram.gif
Human finger provides a path for coupling the sensor to earth ground, which increases the capacitance CSelf. Liquid covers the touch surface also provides a path for coupling the sensor to the surrounding ground or other components, which increases the capacitance CSelf.
Figure 3. Influence of Liquid on Self Capacitance Method