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FDC2214EVM: Increasing sensitivity

Hendrik Steenland
Prodigy 80 points
Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC2214, ALLIGATOR,

I am working with the FDC2214 development board and it senses people beautifully. However weird it may sound, I am trying to use it to sense a small rodent (mouse) when it interacts with a metal cage. The hope is to develop a product that can sense when rodents interact with their cage lids and potentially develop a commercial cap sensing mouse trap. The problem of course is that the cage lid surface area is much larger than the rodent. So when the rodent touches the lid the capacitance of its body should be added to the capacitance of the cage lid, however i suspect i am just out of range of the sensor. I was slightly shocked by this since this ADC is 28 bit and can detect as low as 1ml water drops. What i cant understand is why it cant detect a 25 gram mouse (bodies are 90% water after all)  through the pads of his feet. I noticed in the documentation that there are options for differential sensing and changing the current of the sensor. Would this be a good place to start or should i think about changing the tank LC resonator circuit? Any advice would be appreciated

Cheers

Hendrik

  • 0
    TI__Genius 13335 points

    Hello,

    Can you comment on how the sensor is connected? Is the cage connected as the sensor?

    Thanks,

    Luke

  • 0 in reply to Luke LaPointe
    Prodigy 80 points
    Hi Luke
    yes the cage is connected as the sensor... that is connected via an alligator clip to the through hole on the board (near the sensor pad). so the logic as i understand it is when the animal touches the lid its skin will be a dielectric with the body(fluids) acting as the remaining plate and a virtual ground .

    Cheers
    H
  • 0 in reply to Hendrik Steenland
    TI__Genius 13335 points

    Hello,

    I would expect that if the cage is connected to one of the INxA/B inputs that it would function as a nice capacitive sensor. However if you are not seeing much signal change I would double check your sensor component values to make sure the capacitor of the LC tank is not too large. The smaller the better, but I would try to stay above 33pF or so to keep a stable oscillation frequency. If you increase the value of capacitance too much then it will attenuate the signal you are seeing. Secondly I would make sure that it is indeed connected and there is not an issue with a loose / poor connection. You could also use an ohm meter to make sure that the cage is not accidentally grounded. Let me know how it goes after checking these few things.

    Thanks!

    Luke

  • 0 in reply to Luke LaPointe
    Prodigy 80 points

    Hi Luke

    thank you for your help,

    the caps that came with the dev board are at 33pF according to the spec sheet, so are as low as you suggest

     I was also wondering if the distance between the the tank and the sensor matters. 

    Finally if I am attaching a cage lid would it be advisable to cut off the small sensor pad from the dev board?..see fig

    cheers

    Hendrik

  • 0 in reply to Hendrik Steenland
    TI__Genius 13335 points
    Hi Hendrik,
    Yes you can cut the board there and then use the exposed holes on J4-J7 points to connect your sensor. However I will emphasize an important point. If the cage/sensor is all one piece (electrically all shorted together) then you should only connect it to one of the inputs (INxA or INxB) to avoid shorting out the FDC driver. This will be a single ended connection similar to the one shown on the front page of the FDC2214 datasheet as well as how the FDC2214EVM comes by default before removing the breakoff section.
    Regards,
    Luke