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FDC1004 Active Shield - How About the GND ?

Louis47356
Prodigy 230 points
Other Parts Discussed in Thread: FDC1004

Hello

In our application there is a distance of 10 cm between the 4 sensors and the FDC1004. We have wired it with a coax calbe using the Active Shield, as shown in Fig. 6, chapter 3.1 Single-Ended Measurement in the document 'Capacitive Sensing: Ins and Outs of Active Shielding' (SNOA926A). But the result is, that the influence of a hand can be seen in the result. Why?

What about the Ground? In the document, as well as in the manual, the GND routing is not specified. If the sensor is on the same PCB as the IC, it is obvious. But if the sensor is connected with a wire - so we have installed a separate GND wire between every sensor and the SHIELD1 pin of the IC. Is that correct, or shall we shield the GND also?

Best regards

Alois

  • 0
    TI__Expert 7400 points
    HI Alois:

    How large are your sensors? Depending how big they are, the 10cm distance between your sensors and FDC1004 is most likely in your intended sensing area. This means that if you are touching the coax cable, the measurements are changing because your sensors are picking up your hand and not from the actual signal line.

    As for GND, typically GND sources will reduce your sensitivity so you want to limit as much GND plane or traces as possible that run from the FDC1004 to your sensors. We use an isolated sensor topology so there is no dedicated GND electrode, the electric fields terminate at it's nearest GND potential, most likely the GND underneath your FDC1004 or close to it. It is possible to SHLD the GND, but then the concern becomes where your capacitance load on the SHLD drivers may exceed the specification of 400pF. You can easily check this with an impedance analyzer.

    Thanks
    -David Wang
    Capacitive Sensing Applications
  • 0 in reply to David Wang11
    Prodigy 230 points

    Hello David

    Thanks for your answer.

    There are 4 sensors, each 1 x 1 cm square parallel copper plates with ca. 0,5 mm rubber inbetween, to form a force sensor. One plate is fixed, the other moving. Once force is attached, the distance between the plates changes and the capacity changes. This is the goal.

    I'm not able to find out exactly if the sensors, the coax cable or the GND cable pick up the hand, because everything seems to be sensitive. But I think that the sensors are picking up in my application.

    Maybe this capacitive method is not optimal usage, and a resistive or piezo force sensor is better.

    Best regards

    Alois

  • 0 in reply to Louis47356
    TI__Expert 7400 points

    Hi Alois:

    Depending on your setup, I do not believe that capacitive sensing is the best method to be used as a force sensor, especially if you are looking at the change in capacitance based on the varying distance in between the plate. The reason why is depending on what the target is that is forcing the plates closer together, that target dielectric material would be influencing your measuring. For example, if your hand if pressing on the CINx electrode and is traveling closer to the GND electrode, then your hand will be influencing the measurements because the electric fields will be projecting on the top and in between the plates....

    I would recommend looking at either a resistive or piezo force sensor because it does not dependent on target material or size. It's based off of only force applied.

    Thanks

    -David Wang

    Capacitive Sensing Applications

  • 0 in reply to David Wang11
    Prodigy 230 points

    Hello David

    Thanks a lot!

    Best regards

    Alois