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FDC1004: Sensor design on 4-layer PCB, submerged in liquid

Love Kull
Prodigy 20 points
Part Number: FDC1004

Hi,

Working on a design for measuring liquid level in a grounded metal tank using the FDC1004. I would like to minimize the size of the sensor in the radial direction and would like to know if it is ok to have the level electrodes and references on opposite sides of the sensor. I've been experimenting with foil and the FDC1004EVM and it works great but before I send the design to a PCB fab house I would love to hear your thoughts! 

I was thinking that it would be nice to have the sensor made out of a 4-layer PCB.

The tank is grounded, do I need to use the OoP design or is the conventional design ok in this case?

Do I need the RE reference? The temperature and pressure of the gas vary a lot.

Do you foresee any other problems with the proposed design(s) below?

See attached images!

OoP design on a 4-layer PCB

OoP design

Conventional design on a 4-layer PCB

Conventional design

Any help is greatly appreciated.

Best regards, L

  • 0
    TI__Mastermind 36975 points

    L,

    The sensors and references on opposite sides of the PCB should be okay as long as the shield layers are between them.

    The main intent behind the OoP architecture is to drive the liquid being measured to a zero potential because the opposite polarities of the sensor and SHLD2. This can make your system less sensitive to nearby grounded objects, and possibly EMI.
    The conventional approach on the other hand forces the liquid to a non-zero potential, which can cause measurements to be sensitive to nearby grounded objects.

    I suspect the grounded tank might interfere with the measurements using the conventional approach because the tank - at zero potential - might skew the non-zero potential of the liquid. This may not be a show-stopper, but it might mean you will need some calibrations or fudge-factors in your calculations.

    In contrast, the OoP-based system the liquid & tank will be at the same potential, which (I think) would give better results and simplify level estimates. 

    The need for an RE will depend upon how the gas properties impact the liquid. If the gas temp and pressure change a lot and affect the liquid in a predictable way, then an RE might be a good idea. If the gas and liquid do not share any behaviors, then the RE may not be particularly useful.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 20 points

    Dear John,

    Thank you for a very informative answer! I now think the OoP design is the way to go.

    Best regards, L