Analog / Digital board layout

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ayzen

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Hi all,

Im in the process of laying out my PCB which has a micro-controller with a radio and a number of analog sensors and I have a few placement questions.

1. My board is 4 layers - the 2 middle one's will be GND and VCC. I have 2 power sources digital and analog. Should the VCC layer be the digital DVCC (2.2V) or the analog AVCC (5V), or just the unregulated battery VCC.

2. I am aware of the fact that im supposed to divided the PCB between the analog parts and the digital parts.
Must I divide across a horizontal or vertical axis across the board, or can I also partition between the 2 sides of the PCB (i.e. one side digital, the other analog)?

3. all my power ICs (i.e. regulators, step ups, etc) should they be in the analog or digital domain or does that depend on what are they powering. i.e.: CPU regulator in the digital domain while the sensor regulator is in the analog domain.


This is the first time im laying out a multi layer mixed signal board, I have a feeling im going to make more than one mistake :roll:

Yadid
 

This topic generates a LOT of discussion. But here's what I do (for what it's worth).

1) You can split your power (VCC) plane into several sections, one for analog, and one for digital
2) I would be more inclined to separate my analog/digital sections across the board rather than top and bottom
3) I put all my regulators in one area, but try to put the analog ones closer to the analog power plane
4) I use a single ground plane (I know, I know, this is probably the biggest area of discussion), but you can add cutouts to the plane to 'steer' ground currents.
5) The fact that you imply you are using switching regulators says that you need to pay additional attention to the tons of noise they generate and how that will affect your analog circuitry.

Probably the best advice I can give is to actually THINK about how currents are going to flow on the board. The WHOLE problem comes down to unwanted(digital) current paths through your sensitive analog circuitry. If you can lay out your board so that the digital ground currents flow back to the regulator without having to run through the analog ground path, you'll be ok.

Hope this helps.

Barry
 

Hi Barry,

Thanks for the information. My main point of concern is the single ground plain.
I have read the some engineers prefer to use a star configuration - i.e separate ground planes (that are only connected to their respective regulator), and each regulator is connected directly to the battery GND.

What is the advantage of having a single plain with cutouts as opposed to the above configuration ?

Thanks,


Yadid




 

Using multiple planes can cause problems for several reasons. One reason is that if you have traces crossing the gap between different planes, you will have lost the closely coupled return path for that trace and may now have a large loop which can radiate (like an antenna) and also have a high ground impedance.

It is very unlikely that your analog and digital sections will be totally separate; you almost always have signal traces crossing from one domain to another (analog-to-digital converters, for example).
 

If you have power ics then you should look at their recommended layout. The issue is that if you use one larcge GND plane the voltage from the regulator might not be correct. Usually the IC behind the regulator must be connected to regulator's output AND gnd. Then you voltage is best specified. That said - you have to split VDD/GND for analog and digital due to the noise coming from the regulators and digital ics. Then since grounds have to be connected you have to use star connection at the GND port.
 

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