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FDC2212: Channels interacting with each other

andy quaas
Prodigy 40 points
Part Number: FDC2212
Other Parts Discussed in Thread: FDC2214, FDC1004EVM

I have made a PCB with an FDC2212 for liquid level sensing.  I have 4 electrodes - two on IN0a/b and two on IN1a/b, but they are on two different water tanks.  (see photos).  I filled one tank completely and it worked as expected.  Then I started to fill the other tank, which is about 12" away, and the sensor reading on the first tank is going in the opposite direction as I fill the other tank!  The FDC is counting higher on the full tank and lower on the empty tank as I fill it with water.  Does that make sense?  It seems almost like there's a setting in there somewhere that's making this happen.  Maybe it was a mistake to connect both tanks to one FDC?  The PCB is laid out just like the EVM.  I have a 2214EVM, but I never thought to try it with both tanks connected to see what happens before I went ahead with all of this.

Here's a photo showing the setup.  The black plastic enclosure has the FDC in it.  You can see all 4 electrodes here.  They are 1" wide and about 13" long spaced 0.5" apart.  The tank on the left is full of water.  The capacitance of the righthand tank did not change while I was filling the left tank.  When I started to fill the righthand tank its value changes as expected, but the lefthand tank started to offset by almost the exact same amount! 

Here is a graph comparing the two tanks' raw FDC output values.  The left tank on the graph is the left tank in the photo above.  The steps in the graph are where I stopped filling the righthand tank!

Any ideas?

  • 0
    Prodigy 40 points

    I did some more testing.  I went ahead and just completely filled up the other tank (right hand side) and it looks like I got a "bump" on the left side, so it went up a bit and came back down.  I might have some interaction between the tanks I guess.  Does grounding the liquid inside the tanks have any benefit in all of this?  I can probably deal with this, but I'd rather find a reasonable way around it.

    Here's the chart showing the two tanks in action.  The upper line represents the left tank from my previous post, you can see the bump while the other tank is shown filling below it.

  • 0 in reply to andy quaas
    TI__Expert 7599 points

    Hello User,

    Our lead applications engineer supporting LDC and FDC is out of office and will return next week. We will review the question and get back to you 21st of October. 

  • 0 in reply to Arjun_Prakash
    TI__Genius 17440 points

    Hi Andy,

    Thanks for your patience while I was out of office. Based on the size of your sensors, it's not surprising that the liquid level in the second tank is affecting the measurement of the first sensor. These sensors will have a very large sensing range (potentially ~20 inches, based on figure 3 in our capacitive proximity sensing app note). It's very possible that the second tank is within the sensing range of the first sensor.

    I don't believe that there's a specific setting that's causing this, but I do recommend verifying that the sensor oscillation amplitude is within the optimal range (1.2-1.8V). You can probe INxA or INxB vs the FDC's GND to make sure this is the case. 

    I don't think it would help to ground the liquid. Additionally I think this behavior has more to do with the size of the sensors and their proximity to the tanks than it has to do with both sensors being connected to the same FDC2214. It's better to use a single FDC2214 for this setup, because then the sensors will definitely not make measurements at the same time and potentially interfere with one another.

    Best Regards,

  • 0 in reply to Kristin Jones93
    TI__Genius 17440 points

    Hi Andy,

    I haven't heard from you in a while, so I'm assuming you were able to resolve your issue. If this is not the case please feel free to reply to this thread or post a new question.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 40 points

    Sorry about that - I was out for a couple weeks then as well.

    The app note there is a little hard to translate to liquid level sensing, especially since I started down this path using the FDC1004EVM, which is a different animal.  I don't have an issue separating the two tanks to two different FDC2212's, but I think it might be smarter to change the dimensions of the electrodes and do some more testing first.  The tanks are quite a bit closer than 20" to each other's electrodes.  The tank thickness is .25 to .5" thick.

  • 0 in reply to andy quaas
    TI__Genius 17440 points

    Hi Andy,

    I agree, you may achieve better results by changing the dimensions of the electrodes. If you need unidirectional sensing, you might also try active shielding: https://e2e.ti.com/support/sensors/f/1023/t/802488#pi320995=2

    Best Regards,