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FDC2214: LC-Tank design

user5139108
Prodigy 30 points
Part Number: FDC2214
Other Parts Discussed in Thread: LDC1041, LDC1051

Hi all,

we plan on using the FDC2214 for capacity measurements at different frequencies and would kindly request your advice.

(1) Currently, I am working on the LC-Tank design for 10kHz. I read this blog post (https://e2e.ti.com/blogs_/b/analogwire/archive/2014/11/21/inductive-sensing-sensor-frequency-constraints).   Does anybody have a recommendation where to find more applicable documentation for the LC-Tank design?

(2) Can anybody provide me with the Rp value of the FDC2214?

(3) Assuming that the LC-Tank would have a frequency of exactly 10kHz, how precise would be the capacity measurements? Would I be able to measure low pF (0.1 pF changes) capacity changes?

Thank you for your support in advance. I am grateful for any suggestions or comments.

Best regards

Niklas

  • 0
    TI__Genius 14270 points
    Hello Niklas,

    Is there a reason that you selected a 10kHz sensor frequency? This is rather low, and you will require a lot of inductance or a lot of capacitance to operate at that low a frequency. If you have a large amount of capacitance, then you will attenuate the capacitance shift. With a very large inductor, you may have a lot of parasitic capacitance which may not be as stable across temperature.

    Some additional information on LC sensor design is also available at www.ti.com/.../snoa930.pdf, although this is focused more in the inductive sensing.

    For question 2, are you trying to determine the Rp value that is appropriate for your sensor design?

    You can use the equation fsensor = 1/(2PI*sqrt(LC)) to assess the shift in sensor frequency with a 0.1pF cap shift. You can refer to the application note www.ti.com/.../snoa944 for resolution capabilities of the FDC2214 - the measurement core is common for both the FDC221x and LDC161x.

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 30 points
    Hi,

    thank you very much for your fast respond!

    (1) Unfortunately I need this low frequency for my measurement. We need to differentiate water, ice and air. My literature suggested that the frequency needs to be lower than 20kHz to measure a difference in permittivity (the lower the better for differentiation).
    Do you have reference designs for measurements at those frequencies? Does TI have recommendations for LC-Tanks with similar frequencies? Or would you say that it is impossible to do in reality?
    (2) I asked for the lowest Rp value, because you wrote in the blog (e2e.ti.com/.../inductive-sensing-sensor-frequency-constraints) “Devices like the LDC1000, LDC1041, and LDC1051 can drive sensors with RP as low as 798 Ω.” Is it the same for the FDC2214?

    Thanks again for your support!
    Best regards
    Niklas
  • 0 in reply to user5139108
    TI__Intellectual 2725 points

    Hi Niklas,

    Have you taken a look at the FDC2214 based frost/ice detection reference design:

    Regards,

    Varn.

  • 0 in reply to Varn Khanna
    Prodigy 30 points

    Hi,

    thanks a lot for the reference. Even though it is a great reading, its design uses a frequency of 6.5Mhz.

    Unfortunately for my measurement I need a much lower frequency. Do you know a reference design, which has used a frequency of 10kHz to 20kHz?

    Thanks again for your support!

    Best regards

    Niklas

  • 0 in reply to user5139108
    TI__Intellectual 2725 points

    Hi Niklas,

    Even though the LDC device can operate at the lower frequencies but these frequencies are usually of lower interest as the discrete inductor and capacitor values get large.

    In terms of architecture of the device lower sensor frequencies will provide a better resolution but we are limited  by the 28 bit word width of 28bits. At these low frequencies, the LDC161x will output sequential codes. While the resolution may be maximized with a low sensor frequency, the maximum useful resolution occurs with fSENSOR between 400 kHz and 6 MHz (with fREF = 40 MHz).

    I am not sure if you have gone through section 2.4 of the following app note but it dives down into details of what trade-offs you need to consider:

    Did you really just want a recommended inductor and cap? if so, please provide more information about your sensor (size, copper thcikness etc.)

    Regards,

    Varn.