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FDC1004: Zero level capacitance changing step wise after hours

Alexander Evers
Prodigy 230 points
Part Number: FDC1004

Hi,

I use the FDC1004 to sense the level in a tube with 70mm diameter. The sensor layout is the same as in the liquid level sensing app note. I only streched the outline to 180mm height and 25mm wide sensor electrodes as well as 10mm distance between the sensing electrodes. Shields are 28mm wide and the second level of shielding is mounted on 2mm rubber in between. The complete sensor is mounted on a plastic tube inserted in a metal tube which has earth potential. Sensor wires are shielded.

Now my problem is, that my level calculation uses equation 2 from the app note 925 to calculate the level but the C_L, C_RL, C_RE are changing from time to time step wise so that my level is totally off.

For example first the capacitances are as follows: C_L = 5,2nF, C_RL = 1,8nF, and C_RE = 1,8nF and then some times (nothing was changed, sensor was left for more than 1h unattended) C_L = 6,2nF, C_RL = 2,1nF, C_RE = 2,1nF.  I do not see where this comes from. Reset of the chip or power cycle some times fix the issue some times the caps stay off. I expect drift but these changes happen to be steps that are finished in less then 500ms (i read means of 50 values sampled with 100S/s)

Do u have any hints for me where to look? Is it good to reduce the width of the sensor as they all are assambled on the same tube? I think of some interference effects? (But even if i deactivate CHA_CIN3 i have these issues. So OoP for channel 3 is no issue.) For now i have enough resolution i can live with less than half of it.

Thanks and regards

Alex  

 

  • 0
    TI__Genius 14270 points
    Hello Alex,

    Are you trying to use the Out-Of-Phase technique? Can you provide a picture of your sensor layout, along with the channel assignments and the device configuration? It may be that you have the shields improperly assigned.

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 230 points

    Hi Chris,

    actually I use both OoP and the conventional approch having the same issues. All measurements below were taken with Human Body more then 2m away. And the 

    These samples were taken at 200ms rate with the Setup as in the App Note. CH1 for Level against CH4, CH2 for ref. in the material also against CH4 and CH3 as enviromental reference also against CH4. The chanels have been set to nearly 0 with the offset.

    For this Test i used the OoP sensor with the layout as in the AppNote:

    Inner Layer: SHD2 || CH3 on top and CH2 at the bottom || CH1 level || SHD2 

    Isolation: 0.2mm nonconductive adhesive tape

    Outer Layer: SHD2 || SHD1 - SHD1 || SHD2 (the SHD1 is one big area covering all measurement electrodes. 

    The Steps at around t=6000 in the graph are strange but i think one can calculate the correction with the other sensors but Equation 2 from the App Note: Level = (C_L - C_L0)/(C_RL - C_RE) is no more valid!

    But my real problem are those steps like the one at t = 4000. This one has no impact on Ch2 and CH3 at all.

    The same behaviour i see with another sensor which is inserted in a metal tube with earth potential. But this one is build als follows:

    Inner Layer: GND|| CH2 at the bottom || CH1 level || GND 

    Isolation: 2mm adhesive tape and rubber

    Outer Layer: SHD1 - SHD1 || SHD1 - SHD1 (here covers the shield the gnd and channel electrode).

    This one was rock solid in signal but every now and then the offset changes with a step like in the graph at t=4000. Ch2 stays the same. 

    For now this is all conductive tape and all this quick and dirty rubish, but how does TI recommend those sensor electrodes to be manufactured? I read here a flex board with only 0,1-0,2 mm spacing between the electrodes and the shield is suboptimal. 

    Have u experience with using sma connectors and coax wire from the electrode and shield pair to the board with the fdc1004? This would be roughly 5-10cm wiring. 

    Regrads

    Alex