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FDC1004: ACTIVE SHIELD - WHAT IS THE INTERNAL PROCESS

Jeff Simpson
Prodigy 30 points
Part Number: FDC1004

If the purpose of the shield is to have the same excitation signal as Cin1 so there will be no voltage potential between them.   1) If something is placed in front of Cin1 does the shield excitation change to have the same affect as the object is having on Cin1. 2) If Cin1 is sensing something in the other direction and a person moves a hand behind the shield. does the shield excitation actively change to incorporate the affect of the hand and have the same overall affect as  the excitation signal as Cin1.  Is this active shielding or a baseline shielding based upon just what is just happening with Cin1.

  • 0
    TI__Expert 6625 points
    Hi Jeff,
    The shield pin is used to get rid of any environmental or temperature effects. To answer your question the shield excitation signal does not change with hand movement or if something is placed in front of CIN1. So if there is hand movement or any other changes to the sensor, only CINx changes.

    Best Regards,
    Bala Ravi

    Please click "This resolved my issue" button if this post answers your question
  • 0 in reply to Bala Ravi
    Prodigy 30 points
    FROM THE SPEC SHEET: To eliminate the CINn parasitic capacitance to ground, the FDC1004Q SHLDx signals can be used for shielding the connection between the sensor and CINn. The SHLDx output is the same signal waveform as the excitation of the CINn pin; the SHLDx is driven to the same voltage potential as the CINn pin. Therefore,there is no current between CIN and SHLDx pins, and any capacitance between these pins does not affect the CINn charge transfer.

    How/When does the SHLDx signal become the same excitation as the CINn pin? Why does it not change to have the same affect as CIN by itself + for example the affect of a hand. Please help me to understand the limitations of the SHLDx pin.
  • 0 in reply to Jeff Simpson
    TI__Expert 6625 points
    Hi Jeff,
    The shield pin is driven by the same excitation voltage as the CINx pin. In a single-ended measurement it can drive 200pF per each shield pin. In a differential measurement configuration, the shield pins are at 180 degree phase shift. The fluctuation of the CINx pin excitation voltage is because of the different drive capacitances. The shield pin can drive 200pF versus the CINx pin can drive +/- 15pF. If the movement of hand is from the shield side (bottom) as opposed to the sensor side (top), the excitation voltage will not move much. We can't share the internal design of the pins. Please refer to Section 4, of the datasheet to get a better understanding.

    Best Regards,
    Bala Ravi

    Please click "This resolved my issue" button if this post answers your question