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FDC2212: FDC2212 Engineering Application Technology Consulting

huangjian-sz
Prodigy 20 points
Part Number: FDC2212
Other Parts Discussed in Thread: FDC1004,

  • 0
    TI__Mastermind 37605 points

    Huangjian,

    A few questions about the analog part of the design:

    1. What are the dimensions of your sensors?
    2. What is the physical spacing between your sensors?
    3. What are your sensor frequencies?
    4. Do the results change for a single channel if all of the other channels are off?

    Just so you know, we recommend the FDC1004 over the FDC2xxx devices, especially for liquid-level sensing.
    The FDC1004 features active shield drivers, which help with EMI management, while many of our customers have struggled with EMC/EMI issues with FDC2xx devices.
    You can find out more about the FDC1004 at the FDC1004 product folder and on the E2E FDC1004 Frequently Asked Questions age.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 20 points

    Hello John Miller

    1. What are the dimensions of your sensors?

    The length of the sensor extending outward is 100mm, and the impedance is 42KΩ。

    1. What is the physical spacing between your sensors?

    The spacing between probes is 9mm。



    1. What are your sensor frequencies?

    We use frequencies at two frequency points: 4.6MHz and 5.1MHz。

    1. Do the results change for a single channel if all of the other channels are off?

    Single channel detection is stable and has no impact。

    May I ask if there are any good suggestions for design besides selecting the

    FDC1004 chip?

  • 0 in reply to huangjian-sz
    TI__Mastermind 37605 points

    Huangjian,

    What is the liquid used in your reported results?

    The single-channel amplitude data looks like it moves between the values of -4500 to -3500, while the multi-channel measurement ranges from 8000 to 14000.
    If the two data sets were taken for the same probe and position, it appears the capacitive sensors may be interfering with one-another.
    You mentioned sensor frequencies of 4.6MHz and 5.1MHz.
    1. Are the two frequencies used in alternating probe positions?

    2. The schematic is a little fuzzy, but it seems to show the antenna connected to INxA and the reservoir connected to INxB. 
        What are the details of the INxB connection?

    3. What are the Qs of the resonant antennas + PCB inductors for the two frequencies? 

    4. What is the inductance and self-resonant frequency of your PCB inductor?

    regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 20 points

    Hi
        FDC2212 Engineering Application Technology Consulting

    1. What is the liquid used in your reported results?

    There are many types of liquids, including polar and non-polar molecules

    1. Are the two frequencies used in alternating probe positions?

    We have tried to verify that channels 2, 4, 6, 8, and 1, 3, 5, and 7 use 4.6MHz and

    5.1MHz respectively, resulting in a slight reduction in interference.

    1. What are the details of the INxB connection?



      1. What are the Qs of the resonant antennas + PCB inductors for the two

      frequencies?

      Our antenna probe is 40K Ω and the inductance is 18uH, but we are not sure about

      the internal circuit of the chip, so we cannot accurately evaluate it

      1. What is the inductance and self-resonant frequency of your PCB inductor?

      We use an inductor of 18uH and a capacitor of 18pF, resulting in a frequency of

      around 5.1M;The inductance is 18uH, the capacitance is 33pF, and the generated frequency

      is around 4.6M.




  • 0 in reply to huangjian-sz
    TI__Mastermind 37605 points

    Huangjian,

    Two potential causes come to mind:

    1. The close spacing may be causing mutual interference between your sensors. Placing shields between the sensors may be one option, but the close spacing between the sensor and a shield may affect the sensors and reduce the sensitivity of your system.
    2. A couple of sensor frequencies have been mentioned.
      Have the design frequencies been confirmed by measurements for a single probe and for multiple active probes?
      You can easily measure the sensor waveforms with a high impedance oscilloscope probe with a leaded 1k resistor between the probe tip and the sensor test point.
    3. One parameter to consider is the inductor's self-resonant frequency (SRF), where the inductor self-resonates.
      Above this frequency, it stops acting like an inductor and starts acting like a capacitor. If your sensor operate frequency is more than 70%-80% of the self-resonant frequency, then it will not work reliably. Since you have said a single probe seems to work okay, this may not be the root cause, but it is something to consider, especially considering item (2) above.
    4. Regards,
      John