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The cable between coil and LDC1000

MASUDA TETSURO
Prodigy 20 points
Other Parts Discussed in Thread: LDC1000EVM

I'm evaluating the LDC1000EVM now with using the coil which I made.

The coil is connected to the coaxial cable(3m), and the coaxial cable connected to the LDC1000.

The coil has approx 19uH and 3.72ohm with including the coaxial cable.

I refer to the EVM Quick Start Guide, and determin the C value(470pF) and the frequency(1333kHz).

After these parameters(also Rp min and max) have been set on the LDC GUI software, it shows the proximity data change when the target distance changes. This means the system seem to work correctly.

But if I touch the coaxial cable, the proximity data changes.

I understand C value has been changed by touching the coaxial cable, this results in the proximity data changes.

Question:

Does anyone have any ideas or any clues to avoid(or reduce) this kind of cable issues?

Should these type sensor not be used with the cable?

Please advise.

  • 0
    TI__Intellectual 1290 points

    Hello,

     

    Can you describe the cable? How many conductors are in the cable? Is there a shield? How did you connect the shield?

     

    Don Liu

  • 0 in reply to Don Liu
    Prodigy 220 points

    Hello,

    I'm planning a similar application where the coil is about 1m away from the sensor. What kind of cable would you suggest to avoid problems as stated by Masuda Tetsuro?

    The (industrial) application would measure around 12 coils. Would you suggest to use 12 times LDC1000 or switching between coils as stated in another part of the forum?

    Gerardo

  • 0 in reply to Don Liu
    Prodigy 20 points

    Thank you for messages.

    I use the cable which has a shield and one signal line. so, this cable can connect only to INA(signal line) and INB(shield) of LDC1000. That's why the output is unstable(The capacitive change affects the INB. ).

    Then, I changed the cable which has the twisted cable (each conductors connect to INA and INB) and one shield(connect to GND).

    This cable is better than the coaxial cable. But the output is still a little unstable when the man touches the cable.

    So, finally, I will make a more smaller PCB coil and LDC board. This will be able to solve the cable problem.

  • 0 in reply to Gerardo Vanoni
    TI__Intellectual 1290 points

    Hi Gerardo,

    You can use twisted pair. For 1M cable length, the oscillator frequency should be 500KHz or lower.

    As for multiplexing, you can use relay or semiconductor switch to sample the coils, provided that you

    1. Wait for the LC oscillation to stablize before reading the data. The stablization time is roughly 0.01*Q/Freq, which should be tried out to insure the error is small enough for your application (Q is the LC network Quallity Factor)

    2. Choose the LC tank capacitor that is much greater than the switch's capacitance. Semiconductor switches can have high capacitance - check their datasheet

  • 0 in reply to MASUDA TETSURO
    TI__Intellectual 1290 points

    Tesuro San,

    The 2-condutor shielded cable is the correct choice. If you ground the shield (connecting the shield to the GROUND pin of the LDC1000), touching the cable should not affect the operation.

     

    Don

  • 0 in reply to Don Liu
    Prodigy 90 points

    Hi All,

    my applications (axial position measure) require a long cable between sensor and LDC1000 (up to 10m). In standard application like this we had used low capacitance coaxial cable but with LDC1000 I also experienced large drift touching the cable. On other side capacitance of standard 2-conductor shielded cable at this distance put the LDC1000 out of working parameter. Can anyone help me to understand if there is any possibility to use LDC1000 with 10m cable?

    Thanks!