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

2.2.3.2 Mutual Capacitance

The mutual capacitance method measures changes in capacitance between two electrodes. When a finger touches the area between the TX electrode and RX electrode, the finger reduces the electric field coupling between them, which reduces the mutual capacitance Cmutual (see Figure 4). This reduction happens because the finger is coupled to earth ground, so the user interaction disturbs the electric field propagation between the two electrodes.

When liquid covers the touch surface, it affects the sensing capacitance in two ways. The first effect is that the liquid couples to the surrounding ground on the sensor PCB, which reduces the electric field coupling between the RX and TX and reduces the mutual capacitance, Cmutual. Because liquid has higher dielectric constant than air, the second effect is that the liquid on the area between the RX and TX electrodes increases the electric field coupling between the electrodes, which increases the mutual capacitance. The increase of Cmutual causes the measurement result to go in the opposite direction of a touch, and this opposite result is called a "negative touch". This "negative touch" behavior helps to prevent false touch detection when liquid is present.

For a fixed mutual capacitance sensor design, the overall effect of liquids depends on:

  1. The amount of grounding path around the sensors. Liquid couples the sensor to larger grounding path reduces the mutual capacitance and causes the measurement result to go the same direction as a touch.
  2. The amount of liquid that is present on the touch surface, which determines how much the mutual capacitance increases and how much the measurement result goes in the opposite direction of a touch.
TIDM-1021 tida-1021-liquid-influence-on-mutual-capacitance-method-a-block-diagram.gif
When a finger touches an area where a TX electrode meets RX electrode, it reduces the capacitance Cmutual between TX and RX electrodes. This is because the human fingers are coupled to earth ground so the user’s interaction has the effect of disturbing the electric field propagation between the two electrodes.
TIDM-1021 tida-1021-liquid-influence-on-mutual-capacitance-method-b-block-diagram.gif
1. Liquid is also coupled to ground or other components around the sensor. When liquid covers the area where a TX electrode meets RX electrode, it also reduces the capacitance Cmutual between TX and RX electrodes.
2. Liquid has higher dielectric constant than air so by having liquid on top of the area between the RX and TX it increases the electric field coupling between RX and TX which increases the capacitance Cmutual.
Figure 4. Liquid Influence on Mutual Capacitance Method