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Original question:

LDC2112: Mechanical and resonant impedance(RP)

LDC2112: Skin depth

Alex Wu2
Intellectual 260 points
Part Number: LDC2112

Hi Kristin

If you think SUS304 is hard to make material, we consider switching to copper or aluminum.

I use excel to calculate as below, please give me some suggestions.thanks.

1. The table uses copper as the material. The recommended skin depth is 3 = 20.7 * 3 = 62.1um, which is the thickness of the conductive material copper.

Do you have a recommended distance for copper skin depth? It is same as aluminum?

I increase the material thickness, the excel formula will not change.

The percentage of current higher is better?

2. The table uses aluminum as the material. The recommended skin depth is 3 = 26.1 * 3 = 78.3um, which is the thickness of the conductive material aluminum.

The recommended skin depth for aluminum is  5 = 26.1 * 5 = 130.5um, which is the distance between the coil and the conductive material.

3.The following is a more detailed plan 2 picture, FYR.

  • 0
    TI__Genius 17440 points

    Hi Alex,

    1. Let me clarify a few things about skin depth. This is a property of the conductive material that is affected by the frequency of the AC current flowing through it. For higher AC frequencies, the skin depth is shallower, which means that most of the current is flowing closer to the conductor's surface. The skin depth is also affected by the resistivity of the material, which is why it significantly changes between different materials, even if the frequency of the AC current is the same. 

    Because the eddy currents in inductive sensing are AC currents, it's important that we take the skin depth into account when designing a target. If the target is not thick enough to support most (>95%) of the possible eddy currents, then the target response will be reduced. You can only change the skin depth by changing the sensor frequency of your LC tank sensor. Otherwise, you must change the target thickness to support a given skin depth.

    In the calculator tool, changing the sensor frequency will change the skin depth (because of my explanation above and the formula found in the tool). Changing the thickness of the material will not change the skin depth itself, but it will change the number of skin depths. The number of skin depths is just the target thickness divided by the skin depth. If your target thickness is already greater than 3 skin depths (like it is in your example), then increasing the target thickness will not significantly increase the target response. Your target only needs to be at least 3 skin depths thick, which is equivalent to ~95% of the possible eddy currents.

    Copper is slightly less resistive than aluminum, so copper will have a slightly smaller skin depth than aluminum. 

    2. The skin depth is not the distance between the target and the coil. It is the depth that the AC currents penetrate into the conductive target. The minimum target thickness for an aluminum target at this sensor frequency (9.88MHz) is 3*26.1um = 78.3um.

    3. This stackup looks good to me, although I do suggest using a copper or aluminum target if possible. 

    Best Regards,