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FDC1004EVM: Huge offset at the Reference Environment (RE) electrode

P Borger
Prodigy 40 points
Part Number: FDC1004EVM
Other Parts Discussed in Thread: FDC1004, TDC1000

Hello,

Im experimenting with the EMV to create a level sensor as described at tidu736a.

We have made a sensor FPC as described at tidu736a, whith the 2 reference electrodes exactly same size.

We set the measurements to differential as described at tidu736a (see screenshot below).

However, I see a very large capacitance of about 4.5 pF at the environment reference electrode (see screensot below). Even when I disconnect the sensor FPC the capacitance is about 2.5 pF. When I switch over to singel ended mode by setting CHB to DISABLED the capacitance is less than 1 pF.

When I put my finger on the environment sensor the capacity further increases, as expected.

I measured the shield and sensor signals with an oscilloscope. I saw that the exitation of the SHLD1 and SHLD2 is in antiphase, as expected. The exitation of the 3 sensors is in phase with SHLD1, as expected.

I noticed that at the FDC1004 datasheet figure 16 also the reference capacity is large, see screenshot below.

Do you have an explanation for this high lreading, is it normal?

How must I take this high value into the fluid level calculation (equation 2 in tidu736a)?

Regards, Paul Borger.

  • 0
    TI__Mastermind 23810 points
    It looks as if you are getting similar results as the TI design, and that number should still work with equation 2. Note that you may need to do a total gain adjustment with that equation as well.

    Possibly the additional capacitance is impacted by coupling of the nearby level sensor.
  • 0 in reply to Clancy Soehren
    Prodigy 40 points

    Hello Clancy,

    My results indeed look like the figure 16 of the FDC1004 data sheet.

    And of cource, in the formulla any number will give a numeric result, but a large value as measured will result in a negative height.

    Of course, I can play with multiplication factors, which can be negative, to correct this, but I wonder if the desired ambiant compensation still works correctly.

    What I do not understand at all is why this very high value is measured. Any near trace or electrode can of course influence the result, but 4pF? Even the level sensor itselve has less than 1 pF when no fluid is near.

    Remember that when I disconnect my sensor FPC, the value still is about 2pF.

    Attached below I send you pictures of our sensor FPC. It was designed as a very close copy of yours. I wrote the signals on the surface after verifying the connections with a multimeter.

    Meanwhile I noticed also that at figure 3 of tidu736a this high level for the environmental sensor is not seen. Actually this picture is exactly what would be expected. I wonder what you did to achieve this, there is no explanation provided in tidu736a.

    Regards, Paul Borger.

  • 0 in reply to P Borger
    TI__Mastermind 23810 points
    P,

    I was able to find one of these sensors, so I am planning on running a similar experiment to verify your results. I will give you another update by the end of the week.
  • 0 in reply to Clancy Soehren
    TI__Mastermind 23810 points
    P, My apologies for the delay! I am still working on this.
  • 0
    Prodigy 40 points
    Hi Clancy,
     
    OK, understood, no problem.
    Meanwhile I got some progress.
    I think the problem lies in the layout of the small PCB that holds the CDC and the sensor PCB. The channel 3 track runs closely to other signals that influence the capacitance. I could modify the small PCB and noticed significant improvement, but still too much stray capacitance to perform a good measurement. My target has very low Epsilon-r and not conducting, so my full scale readout is only a few pF.
    Now I’m experimenting with ultrasound. TDC1000_7200. First impression is that it works fine, though I noticed that fluid surface must be parallel to the sensor otherwise the reflected wave is not send back to the sensor.
     
    With kind regards,
     
    Paul Borger
    Electronic Engineer | ED-12
     
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  • 0 in reply to P Borger
    TI__Mastermind 23810 points
    Yes, the TDC family can be used for liquid level sensing as well, and the signal strength will be strongest when the fluid surface is perpendicular to the ultrasonic sensor.

    If you want more help with that solution, please start a new thread so that our TDC focused engineers can help.