One of the issues with planar eddy-current sensors is the placement of the
current return for the primary winding. Often the ends of the primary
winding are spatially distant from one another, which creates an
extraneous and large inductive loop that can influence the measurements.
A sensor geometry featuring a primary winding that reduces the effect of
this inductive loop is presented. The primary winding may include a
plurality of parallel extended winding segments. The segments further
include adjacent individual drive coils. Current flows through individual
drive coils in an alternating fashion. Current flows through adjacent
drive coil portions in a common direction, thereby imposing a spatially
periodic magnetic field with at least two periods.