An AC current flowing through an inductor will generate an AC magnetic field. If a conductive material, such as a metal object, is in close proximity to the inductor, the magnetic field will induce circulating eddy currents on the surface of the conductor. The eddy currents are a function of the distance, size, and composition of the conductor. If the conductor is deflected toward the inductor as shown in Figure 8-1, more eddy currents will be generated.

Figure 8-1 Metal Deflection

The eddy currents create their own magnetic field, which opposes the original field generated by the inductor. This effect reduces the effective inductance of the system, resulting in an increase in sensor frequency. Figure 8-2 shows the inductance and frequency response of an example sensor with a diameter of 14 mm. As the sensitivity of an inductive sensor increases with closer targets, the conductive plate should be placed quite close to the sensor—typically 10% of the sensor diameter for circular coils. For rectangular or race-track-shaped coils, the target to sensor distance should typically be less than 10% of the shorter side of the coil.

Figure 8-2 Sensor Inductance and Frequency vs. Target Distance. Sensor Diameter = 14 mm

The output DATAn registers (Addresses 0x02 through 0x09) of the LDC3114 contain the processed values of the changes in sensor frequencies.