Analogue and digital on the same board.....

[Buriedcode]

Hi, just a question about ground-plane partitioning.

Note, I haven't tested anything yet, just wanted some peoples input on this before I commit to spending hours making a board :?
I have designed a PCB for a 18-bit ADC and controller chip, running at 12.288MHz and using mainly surface-mount. Now at the moment I've partitioned the design into analogue and digital, in order to minimise clock/switching noise for the control chip.
Heres the strange thing, the evaluation board for the ADC I'm using (layout is in the .pdf for the CS5330A) does have a partition, and its dated 1997. But the evaluation board for the newer ADC, dated 2003 and rated at 24-bit (much more sensitive to noise) 'doesn't' have any partition at all. Also the older evaluation board uses through-hole, while the newer one is totally surface-mount.

I have separated the power with an inductor rated at 40MHz, and I'm using extensive ground planes on both sides of the board. So, really, is partitioning neccesary? Or will ground planes do? I have read up on this and the answers are vague to say the least, everyone agrees that a ground-plane is a good idea, but some people have said that a partition isn't always the answer.

One more thing, the partition (on both sides of the board at the same point) is located under some resistors needed to connect the control chip to the ADC, these are through-hole. If a partition is needed, can I make these SMD to save space? and simply have 4 small traces running along the connection between the ground-planes? Well, one person suggested separating the grounds directly underneath the ADC, but its datasheet warns against this (something about interference couplling to the chips package).

Sorry for the long post, I'll summerize:
-Is a partition needed?
-If so, where should I put it and can I have SMD resistors/tracks running through the connection?

As always, any input is welcome, and I would be interested to hear any experience on the subject, as well as theories.

Buried(in)Code
:roll:

 

[farhada]

Hi Buriedcode,
I have done a lot of mixed signal boards and had different results depending on how good you place your sensitive componenst compare to each other and how well connected the ground pins are to the components.

For the SMD devices, you loose a small area between the PAD and the VIA, in some rare cases, this can play a big role in your S/N ration of your design.

For that frequenct and 18 bits, I would say a simple ground level all over the board, as well as a lot of capacitors on each power pin of the digital devices will help you a lot.

If you can put a small CLC filter before you connect your ADC, it will help you as well.

But, if you decide to split your ground(s) make sure that you only have one point of contact between the 2 areas.

Let me know if you need any more specific description of what I did in split/common ground mode.

Best of luck,
/Farhad