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Multiple FDC1004 devices and shield connection

Ricardo Zurfluh
Prodigy 30 points
Other Parts Discussed in Thread: FDC1004

I need for a gesture control application 9 capacitive proximity sensors, which are connected on a PCB with a hatch fill on top and bottom, which are driven with an active shield signal from the FDC1004 shield output. Since the FDC1004 has only 4 channels, I intend to use 3 FDC1004 devices with using an I2C multiplexer. I'm now unsure how to wire the hatch fill with the shield output. Each sensor is connected with a shielded coaxial cable to the FDC1004 device.

Is there any recommendation? Can I now connect the hatch fill and the coaxial cable to all three devices on Shield1 port (parallel) or is one connection to one device enough? Or should I wire each coaxial cable of the corresponding device to the shield port and additionally the hatch fill connection to one specific shield pin of device 1,2 or 3 (doesn't matter which one)?

Thanks for the support!

  • 0
    TI__Expert 8050 points

    Hi Ricardo,

    In general, it is not recommended to share the same shield between different sensing channels because this could cause interference between the channels. In addition, are you planning to use the hatch fill to shield both the top and bottom of the sensors?

    Regards,

    Yibo

  • 0 in reply to Yibo Yu
    Prodigy 30 points

    Hi Yibo,

    thanks for the quick answer. So you mentioned that it's not recommended to share the same shield between different sensing channels. According to the datasheet on page 12 however this is exactly done, why therefore I'm a bit confused. Why has the board only 2 shield outputs although it has 4 sensor outputs?

    Back to my application. The 9 sensors should be placed on the bottom of a glas plate (transparent conductive layers). The same is planned for the hatch fill. This should be placed around the sensors (top) with also transparent inductive layer. To make the sensor only available for one side application and to shield interferences from the back side, at the lowest layer a metal (GND plane) is placed and between the glas and metal ground plane an insulator is used. The hatch fill on top is then driven with the active shield output from one single FDC1004. Then no bottom hatch fill is needed. Can you recommend this approach? An alternative would be to place instead of a metal GND only a hatch fill in the glas layer (top), drive this with the active shield channel output from one single FDC1004 and build an external driver (simple voltage driver with an op amp) for the single capacitive sensors coaxial cable sensor traces. Is this better?

    Thank you for the support again. 

    Best,

    Ricardo

  • 0 in reply to Ricardo Zurfluh
    TI__Expert 8050 points

    Hi Ricardo,

    Sharing the same shield could cause interference between two or more sensors if they have significant coupling to the shield. But this does not imply that it can never be done. After getting more information about your implementation, I think you can still try using a single shield for the hatch fill around the sensors. Just watch out for any interference during your test.

    It is possible to use a ground plane on the bottom layer. However, a ground plane will reduce the sensitivity, so it is better to keep some distance between the ground plane and the sensors.

    Regards,

    Yibo

  • 0 in reply to Yibo Yu
    Prodigy 30 points

    Hi Yibo

    So let me summarize my approach according to your support. I place the different sensors on a PCB (or later in final implementation on glass as transparent ductive layer) with a hatch fill surrounding the sensors on top. At the bottom of the PCB no hatch fill is placed. Then to shield interferences from the other side, I place a GND plane. Is this what you mean?

    What I also could try (I suppose I only need 4 sensors, aligned by a 2x2array) is to place instead of a GND plane another bottom hatch fill, since I need a proximity distance of around 15-20cm. I fear that this won't be possible with the reduced sensitivity caused by the GND plane. Or instead of placing a hatch fill on top, I only place a driven hatch fill at the bottom. Which one would you recommend? 

    Thanks in advance.

    Best,

    Ricardo