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Remote placement / decoupling

fabu
Prodigy 230 points

The datasheet states that it is possible to place the inductor in a harsh environment to decouple the circuit from the sensor. Is there any information on this?

Also, regarding inductor sizes and shapes. Is there any discussion on the sensitivity of the oscillator vs inductance/number of turns?

Thanks!

  • 0
    TI__Intellectual 2790 points

    It's possible to galvanic isolate the coil from the rest of the circuit, either using a 1:1 transformer in parallel with very little core loss (>95% efficiency) or two series capacitors, one for each differential input. 

    If using capacitors, they should be sized at least 100 times the tank capacitance, so as to not affect the bandwidth of the LC tank.  A very large inductor, ~100uH, should be placed in parallel on the side of the IC to suppress current spikes at the differential nodes.

    Note that either method will introduce an offset in the proximity data due to increased series resistance Rs.  This increased resistance will also reduce the transient response time of the system.

    If you're referring to the sensitivity as the proximity data change over the inductance of the coil, there is no fixed rule since it depends largely on the distance and size of your target:

    1. rule of thumb is the sensing distance should be 50% your coil diameter.  the flux of the coil should be as parallel as possible to the metal target

    2. maximum frequency (5MHz) of the LDC1000.  5MHz = 1/(2*Pi*sqrt(L * C))

    3. Suggested parallel resistance Rp at target distance to be well within Rp Min and Rp Max.

    Thanks

  • 0 in reply to Charles Cheung*
    Prodigy 230 points

    Thanks Charles for your reply. I was mainly referring remote sensing in a sense that I install the inductor 10 or 20 meters away from the PCB board. Was that the intention of remote sensing in the datasheet?

    Thanks!

  • 0 in reply to fabu
    TI__Intellectual 1290 points

    In the case you need the remote sensing, yes, you can use long cables to connect the LC tank to the LDC1000. There are some cautions, though. The resonant frequency should be chosen low enough such that the wavelength (on the long cable) is at least 10 times greater than the cable length. Secondly, the cable need to be of paralle or twisted wire pair. Another caution is that if the coil is placed in a highly electric noisy environment so that the operation of LDC1000 is affected, a common mode choke can be used at the IC side of the cable to suppress the noise.

    Don Liu

    Applications Engineering

  • 0 in reply to Don Liu
    Prodigy 40 points

    Hello,

    reffering to preciding post, where an common mode choke is recommended to reduce electric noise, I want to ask, if it is correct, to

    put this choke between the cable shield and the digital (or analog) ground of the LDC1000 ?

    I'm using the LDC1000 with a short cable, which connects the sensor (sensor frequency 400kHz). I have a quite noisy signal, when I connect the cable shield directly with the grounding of the LDC1000. After putting a suppression ferrit between, the signal get's more clean signal.

    Do you have a explanation for this, how I can improve the signal even more ?


    Thanks a lot,

    Michael