2.1.2.1 Large Local Ground Plane

A way of compensating for this ground shift is to increase C_g by introducing a large local ground plane. The ground plane needs to be large in order to increase the effective C_g. Figure 3 shows the system configuration when the laptop is floating and battery-powered but there is a large ground plane that is connected to the local ground of the FDC2214 EVM through a short wire where:

• C_pg is the fixed parasitic capacitance between the large local ground plane and earth ground
• C_p0 is the fixed parasitic capacitance introduced by the large local ground plane

In this configuration, C_pg and C_g are in parallel which effectively increases the overall capacitance between local and earth ground. Because C_g effectively increases, β also increases closer to 1 which helps mitigate the ground shift effect.

NOTE

C_p0 and C₀ are also in parallel which effectively increases the resonator’s total capacitance.

Due to this increased capacitance, the signal seen at C_s will become diluted and the sensitivity of the system will decrease. However, this also means that the observed ground shift effect is slightly minimized:

For example, if C_P0 is 10 times larger than C₀, then the oscillation frequency equation becomes as follows

where changes in βC_s, the series combination of (C_g| |C_pg) and C_s, will have less of an effect on the oscillation frequency than if there was no large nearby ground plane. However, this is more of a second-order effect – the ground shift phenomenon is more directly dependent on how large the effective C_g is.