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FDC2214: Sensor Antenna

Saumya Shah
Prodigy 220 points
Part Number: FDC2214
Other Parts Discussed in Thread: FDC1004

Hello,

How far you can place the capacitive antenna from FDC 2214 IC in order to detect reliable data? Does the antenna have to be on the same PCB or you can have it remotely and bring wires to the board?

Also, how to decide best sensor antenna shape for proximity sensing?

Thanks.

  • 0
    TI__Genius 17440 points

    Hello,

    My apologies for the delayed reply. The sensor electrode (as opposed to the LC tank) can be placed remotely, but I recommend minimizing this distance as much as possible. The further the electrode is spaced away from the sensor, the more temperature changes and changing parasitic capacitances may affect the sensing system. 

    The shape of the electrode does not matter as much as the total area for proximity sensing. You may find this application note helpful.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 220 points

    Thanks for the response Kristin.

    I am planning to use senor electrode remotely and bring signal down to FDC through 50 ohm coax cable, any issues with that?

    I asked a couple of questions on different thread, I will mention them here:

    1. Is it recommended to use shielding for more focused/uni-directional sensing?

    2. If I use active shielding, it will cut the electric field from one side but does it affect the field on other side from which I am sensing? Will active shielding affect sensitivity of electrode?

    Regards.

    --Saumya

  • 0 in reply to Saumya Shah
    TI__Genius 17440 points

    Hi Saumya,

    1. If you are concerned about interference from unintended targets, then unidirectional sensing could be a good idea. It will not increase the sensitivity of the sensor, but it may improve the sensor's robustness in some applications.

    2. Active shielding will not affect the sensitivity of the electrode. Passive shielding (like a solid ground plane) will decrease the sensitivity of the sensor. 

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 220 points

    Hi Kristin,

    How the sensitivity differs if I use squared pattern antenna compare to rectangle antenna as shown in the attachment? I want detection range to be about 30-35 cm so according to the chart my antenna area should be around 30 cm^2.

    Due to space constraint I would have to use probably rectangular antenna pattern, but how the distance range will work out?

    Thanks.

    --Saumya

  • 0 in reply to Saumya Shah
    TI__Genius 17440 points

    Hi Saumya,

    The shape should not affect the sensing range. Only the overall sensor area should affect the sensing range. The rectangular shape should be fine.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 220 points

    Thanks Krisitin. 

    Any issues in connecting sensor antenna remotely and bringing signal to the board using 50 ohm coax cable?

  • 0 in reply to Saumya Shah
    TI__Genius 17440 points

    Hi Saumya,

    Please see my recent post here: https://e2e.ti.com/support/sensors/f/1023/t/828503

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 220 points

    Thanks Kristin. I saw your answer on the post you mentioned. 

    What would be the best method to connect coax cable to antenna? Does it have to be through a connector?

    Should I keep shield of the cable floating or should it be connected to my PCB ground?

    If I have 4 sensors, is it better to have FDC separately for each one or connect 4 sensors on different channels of same FDC? Ignoring channel scanning delay, is there any other issue doing that?

    Regards.

    --Saumya

  • 0 in reply to Saumya Shah
    TI__Genius 17440 points

    Hi Saumya,

    If you need active shielding, I would recommend using the FDC1004 instead, which has build-in active shield drivers.

    I do not recommend connecting the coax cable's shield to ground. This could make the sensor less sensitive.

    I recommend using four channels of a single FDC device to minimize part-to-part variations.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 220 points

    Thanks for your response.

    Which one of the following is better approach for capacitive sensor:

    1. Having 1 PCB multi-layer, on the primary side all of the electronics then ground plane and then copper plane on secondary side which will act as a sensor. I am aware that having ground plane near by will affect the sensitivity but in this case at least FDC will be placed very close to the sensor.

    2. Having PCB remotely and bring the signals from sensor plate to the PCB using coax cable. In this case, there won't be any ground plane available near the sensor but the distance to FDC will be increased.

    Could you suggest which one is better than the other?

    Thank you.

    --Saumya

  • 0 in reply to Saumya Shah
    TI__Genius 17440 points

    Hi Saumya,

    I definitely recommend avoiding coaxial cables if possible. If you can recess the ground plane around the sensor, then this is the better option.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 220 points

    Hi Kristin,

    I kind of have to use coaxial cable but I could reduce the distance from sensor to FDC. Currently I am planning to use 6 inches coax cable. I am guessing adding cable will offset the capacitance but won't affect the overall sensitivity of the sensor. Is that right?

    Regards.

    --Saumya

  • 0 in reply to Saumya Shah
    TI__Genius 17440 points

    Hi Saumya,

    Significant increases in the overall sensor capacitance will decrease the sensitivity to small shifts in capacitance. If you don't ground the shielding of your coaxial cable, a 6 inch cable should be fine. 

    Best Regards,