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Assistance regarding proximity sensor, FDC2112

Rasmus Fristrup
Prodigy 40 points
Other Parts Discussed in Thread: FDC2112

Hi, 

I am currently working on a keypad for a customer, of which a FDC2112 proximity sensor is sought to be used, in the application note it is mentioned that for a single channel, a copper pour of 20,9 mm x 13,9 mm is to be used for the IC. This raises a couple of questions for me.

- Is this area the bare minimum amount for utilization of said IC? 
- Must this area be rectangular? 

To expand on the second question, I've been doing some research from Cypress Capsense, it seems that according to their design rule paper, a capacitive coupling can be achieved by make an enclosing ring around the object of interest. For now the product / object will be using FPC (flexible print) and it's therefore neccesarry to avoid large areas of copper beneath this. 

For reference I've attached a picture that *should* document which copper ring I'm mentioning

The darker area that is encompassing the button area is going to be connected at the end of the ZIF cable to create a ring. 

Please excuse me if I'm in the wrong forum, I'm merely trying to get some inputs regarding do's and dont's. 

  • 0
    TI__Expert 6480 points
    Hi Rasmus,

    When you say the copper flood with size 20.9mm x 13.9mm, do you mean the sensor size as specified in the datasheet? That was just an example of a capacitive sensor that could work with our device. It is not a minimum size nor does the sensor have to be rectangular.

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Prodigy 40 points

    Hi Rachel,

    Yes the copper flood area is the one specified in the datasheet. But it's good to hear that this isn't the hard lower limit, but I still wonder if said copper pour on the drawing i attached would suffice, would anyone be willing to chime in?

    Additionally, if the ring is to be closed, there's unfortunately going to be tracks placed right under the ring where the flat-cable begins, I can't really decide if this will ruin the purpose of the copper pour entirely, or not, would it maybe be better to not close it, but rather make it an elongated strip? 

    Finally the customer wants to install this product in an aluminium enclosure, with a metal casing surrounding it, this could also potentially degrade the field lines to the point of where almost none of them will interact with the palm or finger of a person, these questions are of course only speculations, as I dont have any access to some simulation tools for verification of this hypothesis or not. 


    Thank you very much for the answer supplied thus far.

  • 0 in reply to Rasmus Fristrup
    TI__Expert 6480 points

    Hi Rasmus,

    I've got a couple of clarification questions:

    1. Is each one of the circular rings representative of each button on the keypad? If so, what is the large rectangular ring that goes around the whole FPC? Is that used for some sort of initial proximity system wake-up?

    2. If #1 is as I expect, then what is the purpose of the ring? Is the ring connected to the large rectangular ring?

    A metal enclosure would definitely limit the ability of sensing through it. If the customer is looking to build a keypad through a metal enclosure, we would suggest using our inductive sensing devices (LDC) instead. Let me know if inductive sensing would be of interest!

    Thanks,

    Rachel

  • 0 in reply to Rachel Liao
    Prodigy 40 points

    Hi Rachel,

    1. The large rectangular ring, is the one that encompasses all of the buttons, this is the one that is in question for proximity sensing. It serves no other purpose than making, as you wrote, an initial proximity system wake-up

    2. No, if we're speaking of the same thing, these smaller rings are in fact, a part of the button itself.  

    That's also what i anticipated. The only real viable option, as i see it, is using an ABS enclosure, or any non-metallic enclosures. 

  • 0 in reply to Rasmus Fristrup
    TI__Expert 6480 points
    Hi Rasmus,

    So the main function you are trying to implement with the FDC device is the proximity system wake-up? Are the small copper-filled circles also capacitive sensors for the FDC to determine when a button has been pressed?

    Yes, if the customer would like to use a metallic enclosure instead, please suggest the inductive sensing devices (LDC) to them.

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Prodigy 40 points

    No the buttons are not proximity activated, they are only pressure activated. 

    Thanks, Rasmus.