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RE: FDC2214 Sensor Filter Bank

Christina Seeholzer
Prodigy 50 points
Other Parts Discussed in Thread: FDC2214, FDC2214EVM

On the GUI Configuration Page, under Sensor Properties and Input, the values for the parallel inductance and capacitance look like they can be modified, although it seems the onboard L-C tank for CH0 (L4 and C17) are fixed. Can the values for the LC tank be changed with the GUI? I want to be able to read a range of capacitance values, so can the measurement range be modified by changing the reference capacitor values? If so, how can I determine the range of measurement? In the datasheet, it does mention the maximum input capacitance is 250 nF with a 1 mH inductor. I'm just trying to wrap my head around between the GUI and whats physically on the board. Thank you for your help!

  • 0
    TI__Genius 14270 points
    Hello Christina,

    You can type directly into the L and C fields to change the values in the GUI. The range of measurement is going to be a function of the shift in capacitance, using the equation fsensor = 1/(2PI*sqrt(LC)). You need to make sure that the sensor frequency is within the operating range of the FDC. Also, the sensor Rp must be in the design space (>1kohm).

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 50 points

    Thank you Chris. What I had been doing was measuring fixed capacitors and I was only able to measure capacitors between 1000 and 3300 pF, anything greater than 3300 pF was not returning data that made any sense (using the Data Streaming feature in the GUI). So at maximum input capacitance, the range of measurement would be 10 kHz to 10 MHz? Also, with the MCU detached, is there a way to change the values of L and C (with registers, etc.) or do they default to the onboard LC values? I will keep design space parameters in mind. 

  • 0 in reply to Christina Seeholzer
    TI__Genius 14270 points
    Hi Susan,

    The FDC2214 doesn't really know what L and C is attached to the INx pins. All it knows is the sensor frequency; you can then derive the L or C based on the device output code.

    As for the issue of where large capacitance values are not measured - as the sensor capacitance increases, the sensor signal amplitude decreases. You can check the sensor amplitude with a scope; if the amplitude is not within the optimum range of 1.2Vp to 1.8Vp, you can increase the appropriate channel IDRIVE setting.

    Also, you might want to adjust the deglitch filter setting for the device with lower frequency sensors.

    The FDC2214 requires configuration to operate, and does not maintain the configuration after power reset. You will need to reprogram on every power-up. Usually, an MCU is used, although FPGAs or other logic supporting I2C can also be used.

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 50 points
    Hi Chris,

    The source of my confusion was that I did not understand that the FDC2214 and the FDC2214EVM were two separate products and I was confusing the technical documents for both. The EVM has the onboard LC filter bank whereas the FDC2214 is configurable with a custom sensor. Thank you for your help!