Virtual Ground SNR in Capacitive Touch

Virtual Ground SNR in Capacitive Touch  The term  "virtual ground"  refers to the fact that a  finger   placed close to the ...

Virtual Ground SNR in Capacitive Touch 


The term "virtual ground" refers to the fact that a finger placed close to the capacitive touch pad acts as an electrical body which has a finite impedance to the capacitive touch circuit's own ground.




The quality of a touch measurement is correlated with how much the capacitance between the pad and ground changes upon human touch. This can be directly translated to how large the observed change in count value is when a human finger touches the pad compared to the non-touched count value. The goal of hardware design for capacitive touch is to maximize this change when a human finger touches a pad as well as minimize sources of noise.



2.3 External Disturbances 


External disturbances and environmental changes that can affect either the actual capacitance of a sensor or a change in the measured capacitance are listed below.

Some of these can be dealt with by improving the hardware design


  • Radiated and conducted noise from other components, traces and power supply.
  • Changes or noise in supply voltage. 
  • Jitter in the low frequency (LF) oscillator frequency. (Hardware) 



Some are inherent with the working principle of the capacitance measurement and should be handled by software 


  •  Changes in temperature.
  •  Changes in humidity. (Software) 
  •  Changes or noise in supply voltage can also be handled by programming... 

Source: 


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RajEngineer.com - Analog, Digital, Mixed-Signal, PCB, Electrical Design Engineering: Virtual Ground SNR in Capacitive Touch
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