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WEBENCH® Tools/FDC2214: Why does the capacitance of the sensor increase as the object gets closer?

KIM JINSOL
Prodigy 70 points
Other Parts Discussed in Thread: FDC2214


We made a capacitive sensor with two electrode plates and measured the capacitance of the sensor with fdc2214.
The two electrodes form mutual capacitance.
As the object gets closer to the sensor, the electric field of the two electrode plates cancels out, reducing the capacitance.
When measured with an LCR meter, capacitance decreases as the object approaches the sensor, but when measured with fdc2214, the capacitance increases.

1. Why does the capacitance increase as the object gets closer?


2. Is Csense self capacitance or mutual capacitance?

please answer two questions. Thank you!

  • 0
    TI__Genius 17440 points

    Hello,

    Thank you for your patience as we return from our holiday weekend. We'll have an answer for your question tomorrow.

    Best Regards,

  • 0 in reply to Kristin Jones93
    TI__Genius 17440 points

    Hello,

    I've answered your first question here: https://e2e.ti.com/support/sensors/f/1023/t/836429

    Csense is mutual capacitance.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 70 points

    thank you.

    When a grounded object(human) approaches the capacitive sensor, the object absorbs the sensor's electromagnetic field.

    In theory, i think the mutual capacitance is reduced.

    However, the capacitance measured by the FDC2214 increases when the grounded object approaches the sensor.

    Q1. The difference from an LCR meter is that the FDC2214 measures capacitance at resonant frequency of LC tank. What is principle of increasing capacitance when measuring the capacitance at the resonant frequency of the LC tank as the grounded object approaches?

    Q2. Your answer: When any grounded object approaches the sensor, the capacitance will increase because the voltage stays constant and the target distance decreases. >>> Do you mean that the voltage on the sensor is constant? If the electromagnetic field around the sensor changes, does the sensor's voltage change?

    Please answer two questions. Thank you.

  • 0 in reply to KIM JINSOL
    TI__Genius 17440 points

    Hello,

    In the setup you shared, the mutual capacitance between the TX electrode and the RX electrode may reduce. However, the FDC2214 does not measure capacitance in this way. The FDC2214 measures the resonant frequency of the LC tank sensor. Any capacitance on the sensor electrodes (equivalent to the TX electrode in your diagram) will affect the resonant frequency of the LC tank. The human hand adds additional capacitance to GND on the sensor electrode, which decreases the LC tank's resonant frequency.

    If the capacitance changes are small, the sensor oscillation amplitude will stay fairly constant. For large changes in capacitance, the sensor oscillation amplitude may change. 

    Best Regards,