Analog Design: Minimizing Virtual Ground SNR in Capacitive Touch

The quality of a touch measurement is correlated with how much the capacitance between the pad and ground changes upon human touch. This...



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.




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 figure above illustrates a typical capacitive touch button with a finger approaching. (yikes!)

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 typical parameters of the RC oscillator and the capacitance of a pad is listed below.

  • Frequency: 100 kHz - 1.5 MHz 
  • Capacitance of pad: 5 pF - 15 pF
  • Change in capacitance upon touch: 0.3% - 50% 
  • Typical measurement window: 0.2ms - 4ms


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