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Original question:

FDC2212-Q1: FDC2212-Q1

FDC2212-Q1: FDC2212-Q1

Jochen Luger
Prodigy 20 points
Part Number: FDC2212-Q1
Other Parts Discussed in Thread: FDC1004

Tool/software:

Hi John,

about 3 months ago, you proposed the FDC1004 instead of the FDC2x1y with the following reasoning:

"Also, many customers have struggled to get to production with the FDC2x1y parts due to EMI.
Depending on your application, it may be worth considering the FDC1004, a capacitive sensor that doesn't require a resonant inductor.
The FDC1004 has built-in active shield drivers that help with EMI management."

To my understanding, the active shield driver eliminates the parasitic cable capacitance from the measurement.

Apart from that, does it really have a beneficial effect regarding EMI as you said ?

I guess, in the case of the FDC2x1y one would connect the cable shield to GND. Is this worse than having a cable shield driver ?

 

Thanks in advance.

Best regards,

Jochen

  • 0
    TI__Mastermind 26335 points

    Hey Jochen,

    Thanks for considering to use Texas Instruments. The diagrams below give some insight into the parasitics involved with the active and passive shields.  As the shield is a plate in parallel with your sensor traces (which could also be generalized as plates), there is effectively a capacitor between those lines that is dependent on the overlapping area, the distance between the plates, and the dielectrics between them.  For the active shield, assuming your buffer amplifier has sufficient bandwidth and drive capability, the shield should be in phase with your sensing plates and therefore the potential difference should be zero (except when radiated noise couples to the shield).  So long as your amplifier has sufficient drive strength, when noise from EMI impacts the shield, the shield absorbs the noise and attenuates the emi signal traveling in free space. As the shield and sensor lines form a capacitor, the distance between those lines should be tailored to ensure that the frequency of the EMI signals is not passed.  Smaller capacitances will have greater attenuation and filter higher frequency signals.  A larger separation between the plates should decrease the capacitance.

    As with the active shield, the passive shield also is linked to the sensor lines through a parasitic capacitor.  In this case, the shield and sensor lines are not in phase and therefore not at the same potential.  Depending on the capacitance, this might actually shift your expected operating frequency.  Yet again the passive shield should blunt the impact of the EMI so long as some careful consideration is made for the capacitance between the lines.