LDC0851 Shielding from Interfering Metal

Hello Seema,

I have started a new thread to address this question which is a continuation of the discussion from the following thread: e2e.ti.com/.../505676

The question appears to be how to properly shield from interfering metals by using a ferrite shield. This is a common technique for our general purpose inductive sensors such as the LDC161x family and LDC131x family. There is additionally a blog that discusses this technique in more detail: e2e.ti.com/.../inductive-sensing-how-to-shield-from-metal-interference

However, while this technique can also be used for the LDC0851, not all use cases/coil arrangements are supported because of the matching requirements and variability of the ferrite material.

Ferrite recommended: Side-by-side configuration

For example, in a side-by-side coil arrangement, the ferrite film can be applied evenly underneath both LSENSE and LREF sensor channels. In this way, both channels see the same inductance offset introduced by the ferrite and any temperature effects of the ferrite are essentially tracked by both channels and cancelled, so the sensing distance will remain more or less unchanged.

Ferrite not recommended: stacked coil configuration

Now, this technique is NOT well suited for a stacked coil approach. The reason is that applying the ferrite shield to only the LREF sensor side will introduce an inductance offset that may change over temperature or part-to-part and won't be tracked by both channels. Therefore, you would either need a large enough inductance change on the LSENSE channel caused by the metal to overcome the worst case mismatch (very close proximity) or the flexibility to change the ADJ code dynamically based on the temperature and initial calibration (requires system level characterization and temperature reading).

It is recommended to design the system ideally such that either interfering metals are far enough away to not make an impact, or use a side-by-side coil implementation with ferrite, or use a device with a high resolution output (like LDC131x, LDC161x, or LDC1101) to make a decision about the incoming metal. Otherwise, bringing an interfering metal closer to the sensors will change the inductance reading/reduce range and a stacked coil implementation with the LDC0851 and there are only a limited number of options on what you can do.

The only other option is an advanced technique that can be applied if the interfering metal is at a fixed position and NOT dynamically moving. You can skew the coil inductances (add more inductance to LREF sensor) so that nominal inductance with the fixed metal housing present produces a ratio of LSENSE / LREF ≈ 1. This will then effectively cancel the inductance offset by the metal and keep the switching distance similar to the unshielded/matched coil inductance case. This is of course a bit harder to do without either using an advanced simulator tool to calculate the exact inductances with metal present, such as ADS, or by making a few different coil designs with various inductance offsets to fine tune the inductance (easier if you can afford a few different coil iterations). If you decide to go this route, you can use the "Spiral_Inductor_Designer" tab from inductance calculator tool for a rough approximation and iterate the coil design by subtracting inner turns from LSENSE (or by adding inner turns to LREF): www.ti.com/.../slyc137

I should note that these coil designs would only be applicable for the shielded cases. They would perform worse for the unshielded case and for that scenario it is still recommended to use matched coils as you've already done.

Regards,
Luke LaPointe