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FDC2214: Question about layout settings

Will
Prodigy 250 points
Part Number: FDC2214

Dear TI experts,

I am designing an FDC2214 capacitance measurement system with active shielding.

I have some questions about the ground plane settings.

1. The FDC2214EVM has a portion of the ground plane on the power plane (mid-layer 2).

In my design, the power plane is also smaller than the ground plane size.

So, is there any special use for this ground plane, or does it necessary?

power plane of FDC2214EVM

front and ground plane

bottom and power plane

2. My sensing front ends are coplanar parallel plate electrodes, one connected to CHxA and the other connected to the ground plane of PCB.

Will the electric field excited by the sensor in this way be stably emitted from the CHxA electrode to the ground electrode?

And is there a limit to the trace width?

3. The layout guidelines in the datasheet state that all channels need matched trace lengths, but since I've added buffers, the trace lengths are slightly different for each channel, will this make a big difference to the measurement accuracy?

Best regards,

Will

  • +1
    TI__Mastermind 34445 points

    Will,

    1. The mid-layer ground plane is intended to give the best possible ground under the device's input pins.
    The FDC2xxx family is sensitive to EMI at the input pins, hence the reinforced ground.

    2. As long as you follow the recommended design and layout practices, the sensor configuration should be fine.
    Limits on trace width are imposed by parasitic capacitance and loss, and its impact on sensor frequency and Q.
    A wide trace will increase parasitic capacitance, lowering your sensor frequency, while narrow traces will decrease Q.
    Either of these can change accuracy.

    3. If the trace lengths are within a few percent of one another, there probably won't be a big impact.
    The addition of the buffers could have an impact, but that would depend on their design and implementation.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 250 points

    Hi John,

    Thanks for your reply.

    The ground plane of the FDC2214EVM has a thermal relief structure in the thermal pad area of the FDC2214. Can I use a solid connection between the ground plane and the thermal via?

    Best regards,

    Will