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FDC1004: Stateful behavior with certain liquids

Jason McLaurin
Prodigy 10 points
Part Number: FDC1004

Tool/software:

I'm evaluating the FDC1004 for measuring the levels of commercial printing ink. In this application, metal electrodes make direct contact with the ink, and we measure the capacitance between the electrodes. We have done this successfully for many years, but one color of one new ink is giving us intermittent problems:

- Sometimes we get very low capacitance readings, i.e. less than the parasitic capacitance of the sensor

- When we touch the tops of the probes to simulate higher capacitance, the higher readings will "stick"

In both of these cases, the waveform on CIN1 is visibly different from when the sensor is reading correctly. I would like to better understand how the excitation circuit is connected to CIN, and roughly how the capture works, so that I can reason about what might be happening here.

I will attempt to attach scope captures that illustrate how the CIN1 waveform looks different when we're having problems.

  • 0 in reply to Jason McLaurin
    TI__Mastermind 26335 points

    Hey Jason,

    Thank you for sharing those images.  So if you use a different ink on CIN1, does the issue go away?  Is there anything that has changed for this particular design compared to prior designs with the FDC1004?  Could you tell me the electrical properties of the ink including conductivity and permittivity?  Also if you can provide some diagrams of your sensor setup, that might be helpful.  If you are concerned about sharing things publicly, there is an option to send through private message.

  • 0 in reply to Patrick Simmons
    Prodigy 10 points

    Hi Patrick,

    Thanks for the quick response. To answer your questions:

    - We do not have this issue with most inks, and there are just a few new inks that cause problems.

    - We have not used the FDC1004 in the past--we used the built in touch peripheral of our MCU. We're considering switching to the FDC1004 to get something higher precision that's better supported for this application.

    - Re: properties of this ink: the ink causing the problems appears to be slightly conductive. These inks have a permittivity lower than water, roughly in the range of certain types of oil. I don't know the exact constant.

    - Our setup is just two conductive metal probes in the ink, one connected to CIN1 and the other connected to ground. 

    Roughly what is the equivalent circuit between the excitation source and the CINx terminals in the FDC1004? Is the output impedance presented to the CINx terminals constant, or does the driver circuit connect and disconnect a driving source during different phases of the excitation and capture? Roughly speaking, does the ADC measure the edge rates as the signal transitions between low - medium - high to deduce the connected capacitance? 

    I believe that the ultimate solution to this problem is adding a non conductive coating to these probes, which would ensure that we're always measuring the permittivity of the liquid instead of sometimes being influenced by its conductivity.

    Thanks so much for your help!

  • 0 in reply to Jason McLaurin
    TI__Mastermind 26335 points

    Hey Jason,

    My understanding is that there is an excitation circuit that drives the CINx sensor for brief cyclic durations.  Between excitations, the charge that accumulates on the sensor electrode is discharged through a switched capacitor circuit. From the voltage measured across the internal capacitor, the charge can be calculated, and from that charge the external capacitance can be extrapolated.  With the successive approximations from the downstream internal ADC, the error in those measurements decreases.   For a little more information on the FDC1004, you might be interested in looking at this application note, in particular figure 4-1.

    As the ink is conductive, coating the probes seems like a good measure.  Let me know if that solves the problem or if you need additional support.