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FDC2214EVM: FDC2214 Limits and Noise

Abdulkadir Yasin Atik
Prodigy 10 points
Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC2214, FDC1004

Dear Sir/Madam,

I have purchased FDC2214EVM as well as FDC2214. My ultimate goal is using FDC2214 to measure capacitance of my capacitive biosensor. According to the Impedance Analyzer (MFIA, Zurich Instruments) results, my sensor is working in the range of 1000-1200 nF. In the data sheet I saw that the chip is capable of measuring 250 nF with 1mH inductor connected to the LC tank. I have several questions regarding this limit.

  1.  Do I have to use 1mH inductor in order to measure 250 nF capacitance? The limiting minimum frequency of the chip is 10 kHz. If I use an inductor with less inductance than 1 mH, I can use higher capacitances while I keep the resonant frequency above 10 kHz. 
  2. When I connect 250 nF capacitor with an LC tank constituted of 1mH inductor and 33pF capacitor, I am getting results with so much noise. (Average: 250 nF, Standad deviation: 7.0 nF) However, when I use 100 uH inductor and 33 pF capacitor in the LC tank and connect 250 nF capacitor, I get 250 nF average with 4.3 nF standard deviation. With 100uH inductor, I still have noise out of my expectations. However, the fact that noise with 100 uH inductor is less than 1mH inductor is a surprise to me since the test case in the data sheet of 250 nF was with 1mH inductor.

I would be more than happy if you can help me out with these points.

Thank you so much.

  • 0
    TI__Mastermind 40295 points

    Abdulkadir,

    There is no specific value of inductance that is required.
    The 1mH inductor is only an example to provide a reference point in the data sheet.

    With regards to the sensor behavior, the parameters to pay attention to are:

    1. Sensor Rp
    2. Sensor Drive Current
    3. Minimum Sensor Rp
    4. Sensor Resonance Frequency
    5. Oscillation Amplitude

    The sensor LC acts as a bandpass filter, so an LC combination with higher Q will tend to have better rejection of external noise sources.
    In contrast, a lower sensor frequency can also give better noise performance due to the sampling nature the device.

    These two trends can oppose each other, so the challenge is to find the best combination of LC values, Q, and sensor frequency that will meet your requirements.

    The other consideration is EMI. Customers have sometimes struggled to get to a final design because of EMI.

    Because of these factors, we recommend the FDC1004 over devices from the FDC2xxx family.
    The FDC1004 does not use a resonant LC tank circuit on the input, and it has active shield drivers that help manage EMI.
    You can find more info at the E2E FDC1004 Frequently Asked Questions page.

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Thank you so much for your quick response and advices. My sensor is working between 1000-1200 nF. As far as I know FDC1004 can only detect up to 13 pF. Is there a way I can use FDC1004?
    Also, about FDC2214, can I use the chip to detect capacitances over 250 nF if I select an inductor which makes the overall sensing frequency above 10 kHz? I am asking since the datasheet says the maximum capacitance that can be sensed is 250 nF.

  • 0 in reply to Abdulkadir Yasin Atik
    TI__Mastermind 40295 points

    Abdulkadir,

    This is an older part, so the background is unknown on the 250nF max capacitance.
    Note that  the spec is shown as Typical, not a Maximum, which means it may not be a hard limit for the device to function, and have more to do with the chosen fsesnor=10kHz and L=1mH,

    As a guess, if the capacitance gets too high, the oscillator may not be able to drive it due to current limitations, with some additional challenges due to temperature variations.
    But that is just a guess.

    The FDC1004 can measure from 0pF to 115pF by using the internal CAPDAC.

    regards,
    john

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Thanks for your time and answers.