LDC1041 Datasheet, PDF (9/33 Pages) Texas Instruments – 8-bit Rp, 24-bit L Inductance-to-Digital Converter with SPI

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Feature Description (continued)
LDC1041
SNOSCY1 – MARCH 2014
d
Metal
Target
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
characteristics. In Figure 6, the inductance Ls is the coil’s inductance, and Rs is the coil’s parasitic series
resistance. The inductance L(d), which is a function of 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.
Eddy
Currents
Conductance
of Metal
Target Metal
Surface
Distance d
Ls + L(d)
Rs + R(d)
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
this manner the power consumption is reduced to the eddy and inductor losses Rs+R(d) only.
L(d)
C
Rs(d)
Oscillator
Figure 7. LC Tank Connected To Oscillator
The LDC1041 doesn’t measure the series resistance directly; instead it measures the equivalent parallel
resonance impedance Rp (see Figure 8). This representation is equivalent to the one shown in Figure 8 , where
the parallel resonance impedance Rp(d) is given by:
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