In message <004801c0fe3f$648caeb0$0700a8c0@Jerome>, Jerome DUMAS
<[EMAIL PROTECTED]> writes
>Hi All,
>May be a little bit off-topic, anyway here is my question :
>The pcb I'm currently designing features 3 differents ground planes
>(digital, analog audio, and radio-frequency FM).
>I've separated each plane by about 2mm gaps, my question is how to connect
>the 3 ground planes, and where ?
>For now, analog and digital planes are connected by a 2mm pcb wire directly
>under the codec (pcm3002), and the RF ground plane is connected to the
>analog ground plane through a 1nF ceramic cap (in order to avoid RF & audio
>coupling).
>I'm not sure that these connections are the best way to do, does I have to
>connect the 3 planes with a "star" connection ?
>Any help would be really apreciated.
>Cheers,
>Jerome
>
>
This kind of problem is quite complex. A star point is often not a good
idea for this kind of design. If you have a fast clock running between
the digital and RF sections (a synthesizer reference clock for example)
then there should be solid connections between the two ground planes at
the point at which the signal line crosses over to minimise the loop
area for the ground return currents of the clock signal.(i.e. a direct
ground return rather than through the star point)
This may also apply for other signals depending on the design.
Try to visualise the need for the return signal path to physically
follow the outward signal path, and consider interaction with other
signal returns. If the ground return is forced to take a different route
to the outward path then think of this as a loop antenna which can
radiate and pickup high frequency signals. Of course this is more
serious when high frequency clocks are being considered over, say,
audio. Therefore every grounding scheme should be designed according to
the signals involved.
Hope this helps rather than hinders
--
David Want
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