The inductor shape is an important characteristic of the inductor because it determines the shape of the generated magnetic field. A circular spiral generates a more symmetrical magnetic field than other shapes, and is the optimum shape from an inductance verus R_S consideration. In general, it is recommended to use a circular inductor for the highest possible sensing capabilities, unless specific system requirements warrant the tradeoffs of an alternative shape.

For nearly all proximity applications, in which a target is moved orthogonally to the sensor plane, the appropriate shape is a circular sensor that has the best Q (and lowest R_S) for a given area, as shown in Figure 2-2.

Figure 2-2 Axial Sensing with Circular Spiral Inductor

For other applications, different shapes may be more suitable. For PCB based sensors, designing alternative shapes is much easier than wire-wound inductors. For example, rectangular coils can be used to sense accurate movement along the X-axis while having reduced sensitivity to a shift in the Y-axis.