SNOA952 May   2016 FDC2112 , FDC2112-Q1 , FDC2114 , FDC2114-Q1 , FDC2212 , FDC2212-Q1 , FDC2214 , FDC2214-Q1

 

  1.   Ground Shifting in Capacitive Sensing Applications
    1.     Trademarks
    2. 1 Introduction
    3. 2 Mathematical Modeling
      1. 2.1 Different Grounding Configurations
        1. 2.1.1 Earth Ground Referenced
        2. 2.1.2 Floating Ground Referenced
          1. 2.1.2.1 Large Local Ground Plane
    4. 3 Qualitative Test Setup
      1. 3.1 Earth Ground Referenced Laptop
      2. 3.2 Floating Laptop
      3. 3.3 Floating Laptop With Large Local Ground Plane
    5. 4 Conclusion

2.1.2 Floating Ground Referenced

When the laptop is running on battery, it is isolated from earth ground. Because the EVM is powered through the laptop’s USB port, it is also isolated from earth ground. In this scenario, the physical separation of the sensor board (EVM), with small local ground, from the power source, with larger local ground, causes the laptop to also act as a sensor under certain conditions.

Figure 2 shows the system configuration when the laptop is floating and battery-powered. As demonstrated in Equation 6 to Equation 8, when Cg is small, the β-factor becomes small which creates a shift in frequency leading to the ground shift phenomenon.