Mixed signal AGND DNGD separation sugerencies

Community PCB Design
1

Hello, I’m designing my first board and I need to have Analog signals and Digital signals, in particular an Ethernet interface RMII.

The board is going to be 4 layers, with one layer dedicated to ground, another to supply and the top and bottom with signals

The problem is that by the location of components I need to cross the digital signals directed to the LAN with an group of analog signals coming form an DAC, and I don’t have any idea of what ground plane put under this cross section, an Analog Ground plane or an Digital Ground plane.

This is a picture of the cross that is bugging me

https://dl.dropboxusercontent.com/u/471 … 4%3A34.png

The red are analog signal (in top) an the blue (in bottom) are the RMII interface and another digital signals.

I’ve tried to do some research http://www.hottconsultants.com/pdf_file … signal.pdf, and http://www.analog.com/library/analogdia … issues.pdf but I still can’t convince myself of the best strategy to put the founds in this case.

Any suggestion will be welcome, or if somebody known of another forum where I could get an answer.

2

felipec84:
…I don’t have any idea of what ground plane put under this cross section, an Analog Ground plane or an Digital Ground plane.

Those articles are good; as per the first one, you should only have ONE continuous, unbroken groundplane (ie, no split planes), so the question of analog or digital ground is irrelevant. Ideally you would partition your layout so that the high speed digital signals aren't routed close to sensitive analog signals.

However, in your case there are 2 good things you did that reduce the chance of this causing problems:

  • The signals are routed on opposite sides of the PCB, with 2 planes (GND and VCC) between the signals.

  • The analog and high-speed digital signals are run at right angles to each other, reducing the chance of coupling