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FDC1004: Trying to replicate OoP liquid level sense method but reference liquid capacitance increases with liquid level

Mike Sprauve
Intellectual 380 points
Part Number: FDC1004

Hello,

I am trying to develop a custom OoP liquid level sensor system using the FDC1004 based on the reference design. Because of system requirements, I have to have the Reference liquid sensor at the opposite end of the pcba from the FDC1004 chip. I am wondering if the liquid level is affecting the capacitance of the reference level since that trace runs the length of the circuit. I also run a trace on the other side of the PCB to help shield the signal line.

Any and all suggestions would be appreciated. 

  • 0
    TI__Mastermind 18545 points

    Hello, 

    Having the liquid reference trace be submerged by the liquid can cause a capacitance shift as the liquid covers the trace. The trace on the other side of the PCB helps shield from external noise. If you are not as concerned about external noise, you could switch the reference trace and shield trace so that the trace is shielded from the liquid in the tank. This might not completely eliminate the capacitance change, but would limit the effect that it has on the liquid reference measurement. 

    Is the capacitance shift similar to the shift that you see in the level sensor or is it much smaller? 

    Best Regards, 

    Justin Beigel

  • 0 in reply to Justin Beigel
    Intellectual 380 points

    Hi Justin,

    We see the increase on the reference level has the same slope as the liquid level , so that seems to be main issue. One thing to note is that our particular vessel has large air gap between the back of the sensor board and human touch (approx 1/2") so perhaps we can eliminate using the OoP method and use the standard approach. I will still need to run the reference level signal on the backside of the board to eliminate stray capacitance. Is my thinking correct or am I way off base

    Best Regards,

    Mike 

  • 0 in reply to Mike Sprauve
    TI__Mastermind 18545 points

    Hello Mike, 

    If it is the same slope, I think it is more likely that the level sensor is coupling to the reference liquid trace due to their close proximity in your layout. The trace being impacted by the water should have a much smaller response to the liquid being added to the tank. I would move the liquid reference trace to be between the reference and shield if possible similar to how the environment reference is on TIDA-00317

    Alternatively, you could move the trace to be in between the two reference pads similar to how the shields are connected. See below: 

    Best Regards, 

    Justin Beigel