When devices offer different types of measurement capabilities, it’s important for designers to consider which measurement is best suited for their use case.* *Some inductive sensing solutions, like TI’s LDC1000 inductance-to-digital converter (LDC), have two measurement capabilities:

- R
_{P}-measurement: The LDC measures the equivalent parallel impedance of the sensor at its resonant frequency by measuring the energy loss of the sensor due to magnetic coupling with a conductive target. - Inductance (L)-measurement: The LDC measures the resonant frequency of the sensor, which is a function of sensor inductance, also influenced by magnetic coupling with a conductive target.

Some LDCs, such as the LDC1000, even offer both measurement capabilities.

Having these two measurement capabilities leads to a few questions:

- Do you always need to measure both parameters?
- If you only need one, which one should you choose?

Let's compare the two measurement types and explore a few different use cases.

**Sensing range and precision**

The maximum sensing range is similar for L- and R_{P}-measurements and depends primarily on coil diameter, resolution of the LDC and device configuration. A useful rule of thumb for precision applications is that an LDC requires a coil diameter of at least twice the maximum sensing range (for example, we would need a 20 mm diameter coil to measure a target distance up to 10 mm). This applies to both L-measurements and R_{P}-measurements.

**Figure 1. Axial position sensing**

**Reference clock input**

Inductance is measured by determining the oscillation frequency shift when the conductive target approaches the sensor coil. As a result, it requires an accurate and stable reference clock. R_{P}-measurements do not rely on an accurate reference clock and the LDC1000 can perform R_{P}-measurements without an external reference clock. This is an advantage in situations where a reference clock is not available, or where number of wires between the LDC and the microcontroller must be minimized.

**Temperature**

Temperature drift in L-measurements is small compared to the temperature drift in R_{P}-measurements. When using a high-Q sensor, which helps minimize temperature effects, temperature compensation in L-measurement applications is typically only required when you need very high precision over a wide system temperature range.

On the other hand, the resistivity of any metal has a known but significant temperature coefficient, which becomes relevant in R_{P}-measurements. For example, the resistivity of copper changes by 3900 ppm/°C, aluminum by 3900 ppm/°C and iron by 5000 ppm/°C. To account for the change in resistivity, temperature compensation is typically required for most applications that employ R_{P}-measurement.

**Spring compression applications**

Compressing, extending or twisting a spring changes its length, diameter and/or number of turns, which in turn changes the spring inductance. Therefore, measuring inductance directly, rather than R_{P}, is the obvious choice for this application.

**Figure 2. Spring compression measurement**

**Metal composition applications**

Inductive sensing can be used to differentiate between different types of metals. In such applications, an L-measurement provides information on the permeability (μ) of the metal, because the inductance of the system is greater with greater μ of the metal. By contrast, an R_{P}-measurement provides information on the resistivity (*ρ*) of the metal.

As eddy currents flow through the conductive target, the induced electric energy is dissipated based on the value of *ρ*. This is indicated by a change in R_{P}. By generating a table of inductance and R_{P} at a fixed distance from the coil, we can identify different metal alloys. To detect metal composition, we need to measure R_{P} and L simultaneously.

**Metal choice**

Most metal types can be equally well-measured with L or R_{P}. However, there are some magnetic materials where the L response at certain frequencies is significantly smaller than the R_{P} response. For those materials, R_{P} is a more appropriate choice. We will cover this topic in more detail in an upcoming blog post.

**Which measurement approach will you use?**

For most applications, you may prefer the reduced system design complexity of L-measurements due to lower temperature effects. There are two exceptions, in which R_{P} measurements are required: systems in which no accurate reference clock is available and selected designs use magnetic materials as a target. And metal composition detection demands measuring both parameters simultaneously.

**Additional resources:**

- Download this application note for more on LDC temperature compensation.
- Learn more about inductive sensing.
- Read more blogs on inductive sensing design, including sensor frequency constraints.
- Check out WEBENCH® Inductive Sensing Designer.
- Search for answers and get help with your designs in the TI E2E Community Inductive Sensing forum.

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