At 02:56 PM 6/26/01 +0200, Jerome DUMAS wrote:
The advice to read the Signal Integrity list is good.
>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 ?
The answers to this question depend so much on circuit functions that it is
difficult to give a single answer.
Star connections radiating from an area containing the main bulk bypass
capacitor ground pin are common.
>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).
That 2mm PCB wire will cause DRC problems unless the schematic has only a
single ground net for both the analog and digital planes. We've recommended
the use of a virtual short for this purpose. On the schematic, it appears
as a two-pin jumper; on the PCB, it is a footprint containing two pads (or
pad/fill combinations, which can be a little easier to work with in this
case) which have a gap of a few microinches. A design rule allows this
particular part to have such a small clearance between pads. Such a gap
cannot be fabricated, with many layers of impossibility. So DRC thinks that
there are two nets while the physical PCB ends up with one continuous wire.
Another advantage is that the process is schematic-controlled, and you
cannot accidentally short together analog and digital grounds in any
additional locations; DRC would detect this.
As to the cap, it will efficiently couple RF into the analog ground plane,
the opposite of what was stated. Such a cap would be used to couple RF
and/or analog signals while maintaining DC isolation.
To connect at DC or low frequencies while isolating RF, an inductor would
be used.
[EMAIL PROTECTED]
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433
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