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LDC1612-Q1: LDC1612-Q1: Choosing capacitor value using LDCCOILEVM coils B&C

Julio Apps
Expert 3665 points
Part Number: LDC1612-Q1

I want to set up the LDC1612-Q1 EVM with stretched differential coils B&C from the LDC Coil EVM. I am trying to choose a capacitor value to use with these coils. From figure 4 in the following reference guide (www.ti.com/.../snou136.pdf) the could inductance at <2MHz is 30uH. Taking this into account i had a couple of questions regarding the value of the capacitor to choose for the  LC tank:

  1. Above 2MHz the coil inductance begins to increase,should i choose a sensor frequency <2MHz to remain in that fairly linear region of the coil inductance vs frequency curve?
  2. What are the tradeoff's between choosing a higher or lower sensor resonant frequency?

Thank you,

Julio Sanchez

  • 0
    TI__Genius 14270 points

    Hello Julio,

    I assume that you are planning on using Coil B and will recommend based on that coil.

    First, the effective inductance is only going to be ~29MHz; the apparent increase is not real and is due to the additional phase shift of the parasitic capactiance present in the sensor. The peaking you see at 8MHz is due to the self-resonant frequency of the coil. This means there is about 13.5pF of parasitic capacitance.

    So you do not really need to target the 'linear section' of the inductance curve, as the inductance (intensity of the generated magnetic field) is pretty much constant even to within 5% of the SRF

    I would recommend staying below 5MHz for the sensor frequency.

    As for the recommended operating sensor frequency, it depends on the system requirements. If you require an absolute measurement (instead of a relative measurement), then you will need to ensure that the sensor capacitance >> parasitic capacitance. This is a function of the desired measurement resolution - but I would pick 0.2MHz < fsensor < 1MHz unless the target doesn't work well with that sensor frequency.

    As for the tradeoffs for sensor frequency, the primary one to consider is the target skin depth - at lower sensor frequencies the target material should be thicker. Refer to the app-note  for more details on the sensor frequency and also material composition considerations.

    Regards,

    ChrisO