• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 30 subscribers
  • Views 561 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

FDC2214EVM: Non-monotonous capacitance vs. distance behavior

FDC2214EVM: Issues with monotonic operation

Alex Semeniuk
Intellectual 300 points
Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC2214

Hello community!

We observe with non monotonic response of fdc2214evm, when monitoring gradual displacement of a copper foiled cube  (4 inch x 4 inch x 4inch ).

Basically as the distance from the sensor increases, capacitance does not change or even increases!

The symptoms are close to those described here:

The pictures below depicts out setup:

4 sensors 4 inch x 6 inch are connected via ~1m long triax cables to fdc2214evm. the outermost shielding of triaxes were grounded.

We tried connecting them to In B or leaving floating, but that gave no improvement.

In the reference position the distance from the plates to the cube is ~ 3 cm in all directions.

We carefully monitor oscillating voltage to be below 1.8 V under all conditions.

Could you please help to solve this issue?

Thank you,

Alex

  • 0
    TI__Genius 17440 points
    Hi Alex,

    You mentioned that you carefully monitor the oscillation amplitude to be below 1.8V at all times, but is it also above 1.2V? Below 1.2V the SNR will significantly decrease, which could contribute to this issue.

    In addition, you have fairly long cables leading to your sensors. There is a finite distance that the sensors can be separated from the FDC device, based on the sensor frequency. What sensor frequency are you operating at?

    Best Regards,
  • 0 in reply to Kristin Jones93
    Intellectual 300 points

    Hello Kristin,

    Thank you for prompt reply.

    As for your comments:

    1). Yes, I was maintaining between 1.2 and 1.8V.

    2). The sensor frequency (SF) was a bit different in different setups.

    When external triax shielding was connected to InB, SF was between 2.94-3.03 MHz.

    When it was floating, SF was between 4.4-4.55 MHZ.

    Finally, when triax shielding was grounded, SF was  3.78-4.03 MHz.

    Indeed, we use long cables, but unfortunately we cant go shorter.

    You've mentioned that there is a finite distance that the sensors can be separated from the FDC device, based on the sensor frequency.-How do I find that distance?

    Also, can we adjust sensor frequency somehow (for instance change components of LC tank) to accommodate for long cables,without loosing sensitivity and not compromising on system noise?

    Thank you,

    Alex

  • 0 in reply to Alex Semeniuk
    TI__Genius 17440 points
    Hello Alex,

    I think the long cables could be part of the problem. We have a tool for inductive sensing that calculates the maximum distance between the LDC and the sensor, and I tweaked it a bit to estimate the maximum distance for the FDC2214. At the maximum sensor frequency you gave, the maximum distance between the FDC2214 and the sensor should be ~12cm.

    Obviously 12cm won't work for your application, but the good news is that if this is the problem it's an easy solution. Try increasing your inductance value significantly so that your frequency significantly decreases. Bringing your sensor frequency below 1.5MHz should yield roughly a meter of acceptable separation. Less than 800kHz yields a little over 2m of acceptable separation. The change in sensor frequency won't affect your sensitivity or noise floor.

    Best Regards,
  • 0 in reply to Kristin Jones93
    Intellectual 300 points

    Hello Kristin,

    Lets try it! Would you please suggest the values for new inductors? What type would you recommend?

    Do I just install them in place of 18uH ones on the EVM board?

    Warm regards,

    Alex

  • 0 in reply to Alex Semeniuk
    TI__Genius 17440 points

    Hi Alex,

    I recommend using shielded chip inductors like we do in the FDC2214EVM. The value will depend on your capacitance value (both the SMD capacitor and the additional sensor capacitance). The main sheet of the LDC Tools Excel Calculator includes a quick calculator for the resonant frequency of an LC tank. I recommend inputting your capacitance and tweaking the inductance value until it outputs the frequencies I mentioned above.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Intellectual 300 points
    Thank you Kristine!
    Alex