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LDC design / sensor capacitor

Guru 29690 points
Other Parts Discussed in Thread: LDC1312, LDC1612

Hi Team,

I would like to ask about layout of sensor capacitor for LDC proximity application.
Is it reasonable to put sensor capacitor on underside surface of the board as the following?

I would like to put conductive target more closer to the sensor coil to increase resolution, however the hight of sensor capacitor prevent it.

Best Regards,
Yaita

  • Hello Yaita,

    Sensor capacitor can be definitely placed on the bottom surface of the board as pointed out by you.

    The closer the target is to the sensor, the higher the accuracy of measurement.

    However, make sure the sensor capacitor is as close to the sensor coil (inductor) as possible so that the effect of parasitic capacitance is minimized.

    Please let me know if you have any other questions.

    -Best Regards,

     Suhas R C

  • Hi

    I would like to ask about recommended sensor capacitor values.
    The following application note shows the following in page 19.


    http://www.tij.co.jp/jp/lit/an/snoa930/snoa930.pdf

    Could you tell me the background of the minimum value 300pF?
    Is there any concern if we put 100pF with satisfying sensor frequency range of LDC device? 
    I would like to put lower value than 300pF to increase sensor frequency that increases resolution.

    Best Regards,
    Yaita

  • Hello Yaita-san,

    Yes you can use 100pF. We've used down to about 33pF successfully. If you go much below this point then parasitic capacitances start contributing to your tank frequency which are much more variable than a discrete capacitor of NP0/C0G type. The reason we may recommend higher capacitance is for a most stable sensor frequency or to eliminate any capacitive coupling that may occur when you approach the sensor with your hand. 

    Regards,
    Luke LaPointe

  • Hi Luke-san,

    Thank you for your answer.

    In a contrast situation, could you tell me the background of the maximum value 2nF?
    Is there any concern if we put more than 2nF with satisfying sensor frequency range of LDC device (1kHz to 10MHz for LDC1312)? 

    Best Regards,
    Yaita

  • Hello Yaita-san,
    There are a couple reasons not to use too large of a sensor capacitor with the primary reason being that the sensor RP will decrease significantly. The LDC1612 only supports down to 1kΩ sensor RP and realistically it's better to operate at a higher RP whenever possible to increase the Q factor and decrease the noise. The other reason is that we recommend C0G/NP0 type capacitors which are not always available in higher capacitance values for small package sizes.
    Regards,
    Luke
  • Hi Luke-san,

    I appreciate for your support.
    I understand very well.

    Best Regards,
    Yaita