Analog/Digital ground plane

[joe61]

In the Microchip 3008 data sheet http://ww1.microchip.com/downloads/en/DeviceDoc/21295d.pdf it talks about layout considerations when making a pcb (section 6.4). It talks about the use of analog and digital ground planes. What's the difference?

Is it just that a separate pour is done for one vs the other - two separate planes - or is there some difference in how they are made, or ???

Thanks

Joe

[waltr]

Its just a separate pour/plane. The whole idea behind a separate digital and analog ground is current flow and noise. Digital signals switch and pull fairly large amounts of current when they do. But using a separate plane for the analog signals these digital switching current do not disturb the analog signals.

Now, just having separate planes does not mean the analog signals will be quieter. It fact for other reasons a split plane can cause more noise and other troubles (EMI). So there is a bit more to this practice. Look up and read some of the app notes from different manufactures on layout practices for low noise analog and EMI. Microchip has a least one. Analog Device and Liner Tech also have app notes on this.

Also, it depends on how sensitive the analog is. For example, a 1-bit ADC with a 5V Vref has only 4.8mV/bit resolution so won't see noise as easily as a 16-bit ADC with 76uV/bit resolution.

[joe61]

Thanks very much, I'll do some more looking.

Joe

[corrado33]

I agree with waltr.

One of the main things I've learned (when I researched this stuff a few weeks ago  ) is that it is VERY important to keep the digital and analogue traces away from each other.  It's not just the traces that are important though, it's the return paths (of the electrons I'm guessing?  Yes, I know electrons and current flow in opposite directions)  Sure, you ground something to your ground plane, but where does that current have to go?  Back to the battery.   So if you ground a switching digital signal all of the way on the left of your board, and your battery is all of the way on the right of your board, and you have analogue traces in between (even on different layers), that's not good. 

I searched for PCB grounding techniques and read as much as I could.  I also made a tread about it I think. 

I made one on a different site, but basically I'm just posting it because of the PDF I attached to the thread.  Go read that.   
http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/120706-split-not-split-question.html