C

Oscillator

L(d)

Target Metal

Surface

Conductance

of Metal

Distance d

Eddy

Currents

Metal

Target

d

LDC1000

SNOSCX2C – SEPTEMBER 2013 – REVISED SEPTEMBER 2015

www.ti.com

Feature Description (continued)

Figure 5. Inductor With a Metal Target

Eddy currents generated on the surface of the target can be modeled as a transformer as shown in Figure 6. The

coupling between the primary and secondary coils is a function of the distance and the conductor’s

resistance. The inductance L(d), which is a function of sensor to target distance, d, is the coupled inductance of

the metal target. Likewise, R(d) is the parasitic resistance of the eddy currents and is also a function of distance.

Figure 6. Metal Target Modeled as L and R With Circulating Eddy Currents

Generating an alternating magnetic field with just an inductor will consume a large amount of power. This power

consumption can be reduced by adding a parallel capacitor, turning it into a resonator as shown in Figure 7. In

Figure 7. LC Tank Connected to Oscillator

10

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