Sensing distance is directly correlated to the size of the sensor coil. As a guideline, 1um-10cm is a viable sensing range for inductive technology. For most of the LDC devices, including the LDC131x family and the LDC3114, the maximum sensing range can be considered to be 50% of the coil diameter for higher precision applications. For low precision applications, such as a mere detection of metal presence, can be possible up to 100% of the coil diameter. The LDC161x family is the highest resolution LDC device and can add additional sensing range – this device family can sense targets up to 200% of the coil diameter. For simpler LDC metal proximity applications that only require a switch output, the LDC0851 can be considered; it can detect 40% of it’s coil diameter by using the side-by-side coil approach and up to 30% when using a stacked coil approach for size-constrained applications. For more information and tradeoffs between the two different sensor approaches with the LDC0851, see the LDC0851 Stacked Coil Design Considerations.

For a rectangular or elliptical coil, consider the shortest dimension to be the critical dimension when estimating the sensing distance.

Note that the sensitivity is the highest within 20% of the total sensing distance as seen in the below figure. This is because the relative inductance shift versus distance curve is logarithmic in nature and within the 20% cutoff, shorter shifts in distance equate to a stronger shift in relative inductance, allowing for µm level movements in the z-axis.

Figure 5-4 Relative Inductance vs Distance