I am working on a development board (for fun) to use the above micro controller.It has two power pins and two ground pins. One of power pins if analog and one is digital. Should I connect those to separate 3.3V regulators or can it be one regulator? And do the ground need to be separate or can it be combined to one ground plane.
This page is about AGND and digital GND : Grounding of Mixes Signal Systems
But I don't understand it :~ Perhaps it means that to avoid digital currents, you can connect the analog circuit to AGND, but you still have to connect AGND with GND at the microcontroller (but avoid that digital currents influence the AGND).
The power pin for the analog section should be close to the voltage of the digital power pin (like within 0.1V or so), but you can use a LC filter to reduce noise. Normally there is an example schematic in the datasheet.
I looked at the page it appears the general suggestion is to just use a single ground plane and simply run traces so that all of the analog circuitry is in one area and all the digital in another and to keep them separate; which makes sense.
So I will go with one common ground plane.
I didn't see any example schematics for the AVCC in the XMEGA E datasheets, but that's not a big deal.
When you say LC filter I am assuming it is a low pass filter. And why LC instead of RC; is there an advantage of one over another?
So at the moment I will plan on a single ground plane and a LC low pass filter.
And why LC instead of RC; is there an advantage of one over another?
With an LC filter you do not get the drop in voltage that you get with an RC filter. This is because the resistance of the inductor is smaller than the resistance of the R you have to use.
Also an LC filter is a second order filter and the RC is a first order filter. The order of the filter is how quickly a low pass attenuates with increasing frequency.
I would have two separate ground plains and join them at one point.
There is no need for two supplies as shown here but it gives you the idea of two ground planes.
Separate regulators can be necessary if using sensitive analog signals - definitely
good choice for quality audio processing since even slight break-through of digital noise
is easily heard and very objectionable.
Commoning ground at one point under the chip is a standard approach when
mixing digital and analog, most audio DACs/ADCs datasheets recommend this.
There is nothing a seprate regulator will give you that proper decoupling will not give you.
But regulators can be cheaper than a suite of inductors & big capacitors!
Ok, thanks for all the help!
I did finally find the Atmel datasheet for a basic XMEGA E setup and it does have suggested values for a basic power circuit so I will use that as my basis because well its a recommendation so I figure it's a good place to start.
Here is the sheet in case anyone in the future reads this thread and is looking for the same info: http://www.atmel.com/images/atmel-42087-xmega-e-schematic-checklist_application-note_at01080.pdf