Analogue and Digital Supplies (PCB Design)

[David]

Hi all,
In my latest project I have a requirement for the following:

+5V Supply
+3.3V Supply
Ground

+3.3V Analogue Supply
Analogue Ground

How should I go about routing the planes on my PCB design? I doubt I will have room for polygon fills for all supplies/ground so I will have to use the biggest track widths possible. I guess my main question is how and where should I connect the analogue power and ground together? I remember reading somewhere that they should be connected as close to the regulators as possible? I also remember seeing them connected through an inductor?

Any help would be great,
Dave

[Zad]

If you can, feed the power from a "star" point near the regulator and power caps. This hopefully stops any noise generated in the digital part from being transmitted through the analogue side. Consider feeding analogue through a ferrite bead if it is going to be noise sensitive. Obviously fit 100nF caps close to the power pins on any ICs you use.

In this instance, maybe consider using a separate regulator for anlogue and digital sides perhaps? Unless you know what you are doing, use a simple flooded plane ground. Splitting it can help control the currents flowing through it, but you really need experience to know where to put it, or to be able to model the specific pcb.

[jahonen]

For GND, just use single ground plane with components partitioned to digital and analog portions. Do not route digital signals over analog portion of the ground plane. See Ott, H. W., Partitioning and Layout of a Mixed Signal PCB, Printed Circuit Design, June 2001. (PDF file ~ 1.0MB). My advice is not to be a wimp on the ground, use as hefty ground system as possible. A plane (or several) gives best results. It gives a solid foundation for your circuit.

For power, if you use same power supply for analog and digital, don't feed the digital power supplies through the analog section. Do it other way around, analog after digital, with some filtering in between.

Regards,
Janne

[joelby]

Thanks Janne, that's a great reference!

[RayJones]

We built a 14bit digitiser at work where the split plane common point is directly beneath the ADC, as recommended by Analog Devices for the chip we used.

The noise performance is superb, we only see 1 digit of noise with no input. With the signal processing we use we can reliably achieve the full 90dB of dynamic range as expected for our resultant 16 bit data. (ie averaging and digital bandwidth control).

Indeed no traces cross the plane cuts, as advised in Janne's reference.

I suspect if our common point was back at the regulators, we would once again see noise introduced by the potential loop path as alluded by the above document.

Proof is in the pudding as they say.

[David]

Thank you for all the information. Anyone know of any other good sources of information on the problem?

[jpwack]

Hi everyone!

 As always I have a noob question in regard of 2 layer boards: is good to have a Voltage rail plane? is good to have two ground planes (one per layer, two for each "sector" analog, digital or power)?

 I always do a ground plane and a Vdd plane, but I'm having doubts.

Best regards from Chile

[Neilm]

I have found that as long as you pay attention to where the current paths are going to flow a single ground plane over the whole PCB works the best. I had to retest all the instruments my company produced when the EMC rules changed a few years back. In each case going from 2 layer board to 4 layer boards with a common ground plane improved the performance of the instruments.
My referance for when I did this was a book by Keith Armstrong. He has summarised it in a series on articals at http://www.compliance-club.com/

Neil

[scrat]

My referance for when I did this was a book by Keith Armstrong. He has summarised it in a series on articals at http://www.compliance-club.com/

Neil

Since this book was adviced in another thread too (now I can't find which thread it was), I searched for it and found this: http://www.doc88.com/p-14364066512.html. I doubt it is legal...

[Time]

Since this book was adviced in another thread too (now I can't find which thread it was), I searched for it and found this: http://www.doc88.com/p-14364066512.html. I doubt it is legal...

Is there anyway to save this pdf document?  I can't figure it out with this funky browser thing.

[DJPhil]

All seven articles are available on the EMC site, sort of. 
I . . uh . . found a place where you can download them in one pdf, in case anyone needs them.

Hope that helps.

[Neilm]

My referance for when I did this was a book by Keith Armstrong. He has summarised it in a series on articals at http://www.compliance-club.com/

Neil

Since this book was adviced in another thread too (now I can't find which thread it was), I searched for it and found this: http://www.doc88.com/p-14364066512.html. I doubt it is legal...

That document is one of several articals the he has published. They can be downloaded from the compliance club website. Registration is free.

Neil