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TIDM-1021: Reason why the driven shield is hatched

Genius 5840 points
Part Number: TIDM-1021

Hello,

I understand driven shield is needed because we should minimize the ground path around the sensor according to following document.

https://www.ti.com/lit/ug/tidue90/tidue90.pdf

Could you tell me the reason why the driven shield is hatched?

I would like to know the reason plan driven shield is not good. 

Regards,

U-SK

  • Usually just to prevent stress on the board and thus prevent "potato chipping". It looks cool, too.

  • Hi U-SK,

    I like Keith's comment that it looks cool - I think it does too.  I'm not sure what potato chipping is.

    The primary reason the shield is hatched is to reduce the shields capacitance to ground (parasitic capacitance) if it is being driven directly from the MCU CAPT IO pin.  There is a limit of approx 300pF that any CAPT IO pin can drive.  Also, you usually include a small series resistor in series for ESD protection (470 ohm) for that CAPT IO pin.  The large capacitance and resistance create a low pass filter that can slow down the voltage that appears on the shield.  If the shield voltage lags the voltage on the electrodes, then it becomes less effective as a shield.

    Best method is to use external buffer amplifier to drive the shield, if possible.  You will need to check the output load capacitance spec for the amplifier.

  • Usually it is more of a problem with silicon wafers, but a large metal area can cause stresses that warp the PCB, especially when temp cycled.

    ETA Refernces

    "While solid copper pour provides better resistive characteristics, hatched copper pour is used to balance the heat and dilatation on both sides of the board in order to avoid warping of certain substrate.[2] Heating might cause gas bubbles between solid copper pour and certain substrates. Furthermore, it might be possible to adjust the impedance of high frequency traces by using hatched copper pour in order to reach better signal quality.[3]"

  • Good information to have.  Now i know ;)  - Thanks Keith.