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Original question:

LDC2112: One button SPS recommendations

LDC2112: Mechanical and resonant impedance(RP)

Alex Wu2
Intellectual 260 points
Part Number: LDC2112

Hi

I have some design confuse, please correct me if my idea is incorrect, thank you.

The table shows our FPC and material tuning results.
1. The table has 2 RP. RP with no target and RP with target interaction. what do these mean? non-conductive & conductive?

2. What does the target thickness mean? It is material(SUS304 or plastic) thickness?

3. Target distance means the coil to the material surface. I estimate FPC to the plastic button(2.8mm). Am I right?

5. We have 2 plans, which plans you recommend?

  • 0
    TI__Genius 17440 points

    Hello Alex,

    1. I suggest reading LDC Sensor Design for a more detailed explanation of Rp. It is covered at the beginning of the document. Sensor Rp is a function of the series resistance, inductance, and capacitance of the LC tank. Because the target interactions change the inductance of the coil, the sensor Rp also changes. This is why the excel tool shows the Rp value with and without the target. The Rp will be higher without the target.

    2. The target is your conductive material only. The non-conductive material (like plastic) will not be detected by the LDC. In your case, the target thickness would be 0.2mm. Please note that the target must be thick enough or conductive enough to support at least 3 skin depths of eddy currents. This concept is explained in more detail in LDC Target Design. This may be challenging for stainless steel. You may wish to use a more highly conductive material like copper or aluminum. I suggest using the Skin Depth tool in the LDC Tools excel calculator to verify that your target thickness is adequate.

    3. The target distance is the distance between the surface of the sensor coil and the conductive target. In your case it would be the 0.2mm gap.

    5. I don't recommend connecting the rubber to the inner plastic at all. The addition of the ribs in plan 1 looks good. Can you share a closer stackup picture of plan 2? The image is a little hard to see.

    Best Regards,