On Wed, 3 Aug 2022 14:54:38 -0700 (PDT), whit3rd <
whi...@gmail.com>
wrote:
>On Wednesday, August 3, 2022 at 11:03:53 AM UTC-7, sea moss wrote:
>> Two part question (my google skills have failed me for both):
>>
>> 1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no copper below the pads)
>
>But, regardless of the pads, the component's mating metal surface has that
>capacitance regardless, doesn't it? You don't need always to make the 'plates' larger
>than that component feature.
Most surface-mount part capacitance is in the FR4, not in air.
>>
>> 2. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?
>
>The use of guard rings can make such capacitance negligible, if that's required, but...
>trying to simply treat the tiny elements as separable capacitances, isn't a good and useful
>model.
It's often important to know the three capacitances: each pad to the
ground plane, and 3-terminal equivalent capacitance pad to pad.