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  1. #1
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    Resonance concerns for decoupling cap.

    Hi,

    As we place the decoupling cap (MIMCap) at the free space of the layout, there is chance for resonance. So, on which basis I can select the proper size and placement of the cap on the final layout? If there are some helpful documents to understand this concept please mention. Please help.

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  2. #2
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    Re: Resonance concerns for decoupling cap.

    You need "adequate" Q, not maximum Q. You would like
    the SRF of the -assembly- to be well past the operating
    frequencies of interest. Bare MIM caps and bare wiring
    inductance is a recipe for "mystery tank" kinds of chip
    debug adventures. Degenerating the decoupling cap with
    a bit of resistance (e.g. a poly under-jumper) is the done
    thing. Where to put the damping resistance is a judgment
    call - probably between C and supply, with C well connected
    to the circuit block supplies, so the tank energy scrubs off
    back into the source across the resistor. Ground connection
    probably needs to be the most highly respected as it serves
    both signal and power return. Making one plate of MIM be
    part of the ground slab makes a lot of sense, layout area
    economics -wise.



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