Use of stacked coils may be desirable to conserve board space and to prevent false triggering when a target approaches from the bottom. A 4 layer PCB with a thick inner layer is recommended to achieve the best results. It is important to note the direction and polarity of the sense coil and reference coils with respect to each other. The recommended configuration is shown below.

Figure 7-11 Stacked Coil Recommended Connections and Direction