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Ground pouring around analog on a sensitive 6 layer (incl ground plane) board.

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Oct 9, 2009
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I have a 6 layer mixed signal board with the following stackup.

ground plane
Mid layer
Mid layer
power plane

In one corner of the board i have some really sensitive analog circuitry encased within a shield box. I'm wondering how beneficial it is for me to put ground pours on the top and bottom layer (encasing only the analog part) in addition to the internal ground plane. I'm not sure if this will help or deteriorate the performance.

All replies appreciated.

Sensitive analog parts should have their own ground and voltage supply. In any case it must be avoid that fast switching currents flow through the analog ground. A shielding helps to avoid cross talk(magnetic or capacitive), but not against ground or supply spikes or noise. Here is an overview about some rules for mixed signal layout: . If you a mixed supply voltage is needed a hybrid supply will be useful: **broken link removed** .

Enjoy your design work!

The analog circuitry was placed in a corner on the board specifically to avoid digital ground currents. There is a solid ground plane (no split ground) covering the board. It's also nailed down to chassis ground by 2 screws in the vicinity. The analog part has it's own power supplies which are low noise LDO's and the most sensitive part (24 bit ADC over 50 millivolts) is enclosed in a shield box. I'm quite aware of the general guidelines.

I poured both the bottom and top layer around the analog area and stitched both pours heavily to the internal ground plane. Regarding the top and bottom pouring, i'm wondering if there are any things i need to be aware of negatively affecting EMC performance (other then dead copper).

There should be no problem with adding the pours and it should help with shielding. But it will add some small capacitance to any traces nearby.

It would help if you separated the analog and digital portions of the main ground plane with a small gap and then connected them together only at one point with a single trace. That will prevent any circulating digital ground currents from using the analog portion of the plane.
We prefer not to use split planes. But thank you for your reply

We prefer not to use split planes. But thank you for your reply
It's not really a split plane, just a separation of the analog and digital parts of the plane with connection only through a small section of copper trace, which minimizes circulating currents. It's well worth doing and requires no additional layers or processing. Why would you not want to do it?
Our experience has been that maintaining a solid ground plane results in fewer EMC problems then utilizing split ground planes. It was done in the past but problems kept creeping up. Proper board segmention into digital and analog parts has provided better results. Not to say that split ground planes can't be effective, they just tend to be delicate.

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