4 Conclusion

Achieving the desired system performance for capacitive proximity requires an understanding of appropriate system ground design. The ground shift phenomenon can result in unexpected behavior if not properly addressed in proximity detection applications. This issue resides in the fact that the sensor capacitance is in series with the parasitic capacitance between local and earth ground. If the parasitic capacitance between local and earth ground is small, it can act as a second sensor because it represents a variable ground. This can cause a significant frequency shift which will create false detections. Whenever possible the system ground should be referenced to earth ground using a short connection path. Any minor ground shift effects can be compensated for in the software by differentiating between the signal signatures. If referencing earth ground is not possible, then ensuring that the local ground is large enough to increase C_g can effectively create a more stable ground arrangement and minimize the effect of the shift in ground. The tradeoff with this technique is enabled when there is excess SNR available. However, it is important to note that even with the sensitivity decrease, the grounding configurations with the large ground plane can still produce a solid SNR around 10 dB or more.