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LDC1614: Spacing between top and bottom layer of two-layer coil

KIM JINSOL
Prodigy 70 points
Part Number: LDC1614


The sensing distance of one-layer coil and two-layer coil was evaluated using ldc1614.

In theory, since the inductance value of the two-layer coil is larger than that of the one-layer coil, the sensing distance of the two-layer coil should be large.

However, the noise of the two-layer coil was so large that the sensing distance was shorter.

Type 1 is one-layer coil and Type 2 is two-layer coil.

The noise of the two-layer coil is very large compared to the one-layer.

The PCB thickness of the sensor is very small, 0.2mm.

Therefore, it is thought that the coil of the top layer and the coil of the bottom layer form strong parasitic capacitance.
Is this correct? If yes, what is the minimum PCB thickness?

  • 0
    TI__Expert 7599 points

    Hello User,

    Thanks for your post, Its quite possible that the 2 layer board may be adding more parasitics to the system. You will have to use the network analyzer to identify the differences between the 2 PCB coils and conclude if there is a parasitic effect between the layers. Have you had a chance to make the measurements ? Having a stable inductance is key for LDC applications to detect changes in resonance. We highly recommend that the coil construction is decoupled with external parasitics for a stable target movement measurements. Any noise in the system can decrease the dynamic range and can result in reduced target distance resolution. 

    In regards to coil design we do have app note that can guide you in regards to PCB design. http://www.ti.com/lit/ug/snou136/snou136.pdf this is an orderable that you can order and evaluate. 

    As far your question in regards to thickness goes, the PCB design needs to be constructed such that the system can meet the minimum inductance needed for the system to operate.  

    We do have guidelines in terms of coil design and tools available to design can be found here.  https://e2e.ti.com/support/sensors/f/1023/t/295036 

    If you have any further questions please let us know. 

  • 0
    TI__Genius 17440 points

    Hello,

    The sensor inductance actually does not affect the sensing range. The sensing range is based on the outer diameter of the inductor. Typically LDC devices can sense distances up to 50% of the sensor coil's outer diameter. This is discussed in more detail in the FAQ linked below. 

    You are correct that there is parasitic capacitance between the various layers of the coil as well as between the turns. This parasitic capacitance creates the inductor's self-resonant frequency (SRF), at which the inductance and capacitance are equal. This is not a stable operating point for the inductor. We recommend operating any coil below 75% of its SRF. This could be the issue you're seeing here. You can read more about this and how to measure this frequency in section 2.5 of LDC Sensor Design.

    We don't have a minimum PCB thickness; you should be able to design working coils with virtually any PCB thickness. In addition to checking the SRF I mentioned above, I recommend checking the layout of your coil. The current flow needs to be in the same direction on every layer, otherwise the inductor will not function properly. Section 2.3 of LDC Sensor Design has more information about this.

    Best Regards,