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LDC1614: Multi-Layer Coil Scanning

test_user
Intellectual 625 points
Part Number: LDC1614
Other Parts Discussed in Thread: , LDC2114

Hello,

i have a question concerning scanning four coils, placed one underneath the other with four individual channels.

For example four coils are placed one underneath the other (Every coil on a separate pcb layer).

The four coils are not connected, so it's not a multi-layer coil.

Each coil is connected to one of the channels of the LDC1614 and are scanned one after another.

I've attached a drawing which shows the application.

- How do the coils influence each other? 

- Can I detect a deflection of the dome with every channel?

- Can I still detect a deflection if glass is approximately 3 mm thick?

Kind Regards,

Clemens

  • 0
    TI__Genius 14270 points
    Hello Clemens,

    For the first question - if the oscillation frequencies are close, then the sensor will dampen the others. The LDC161x device shorts inactive sensors, which reduces the effect significantly, but you will still have a small interaction if the sensors are physically matched.

    To detect the deflection of the dome depends on the dome size, dome deflection, and PCB thickness. I assume the dome is steel; if it is Al or Cu that would be better.

    If the dome and sensors are 10mm in diameter and the PCB is 1mm thick and the dome deflects by 0.25mm, then you will see the deflection on all sensors. The bottom sensors will have their response attenuated somewhat by the sensors above them, and so they will see a smaller response due to the shielding and the additional distance compared to the top sensor.

    As for detecting through an additional 3mm of glass (or any dielectric), it depends on the size of the dome, the magnitude of deflection, the size of the coils, the size of the sensor, and the conductivity of the metal. A larger dome, larger deflection, larger sensor, and higher conductivity dome will all improve the performance. You should still be able to measure with the example that I provided, although the SNR for the bottom sensor will be lower than the top sensor.

    Finally, with a 4 layer PCB, you will need to play some crazy routing tricks for the return path for the 4 sensors, unless you use wires. If you don't use wires, then the extra routing will reduce the Q of the sensor and degrade the performance.

    I would recommend evaluation with the LDC1614EVM with your sensors to have a better understanding of the performance.

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Intellectual 625 points

    Hello Chris,

    thank you very much for your answer. I think using two instead of four coils would be more realistic.

    I've tested a similar setup (with only one coil) and was able to detect a ~0.5 mm Al sheet movement through 4.5 mm PMMA, using the LDC2114.

    The coil was 10 mm in diameter and the metal sheet 20 mm.

    Do you think the LDC2114 is also an option to sense a ~0.5 mm movement in a distance of 3 mm?