Does placing digital circuitry between the regulators and analog circuitry cause grounding problems?

Question

I'm designing a mixed-signal PCB and have been using this Maxim guide to help me with layout. The guide has instructed me to avoid making cuts to the PCB and instead think about ground return paths. By placing chips in strategic locations and orienting mixed signal chips properly I can avoid analog and digital return currents from overlapping.

I am aware that having digital ground currents run through analog circuitry will cause noise problems. In other words, the regulators (R), analog circuitry (A), and digital circuitry (D) should not be arranged in series like so: R - A - D.

Is the opposite also true? Is analog circuitry performance compromised if its ground return currents pass through digital circuitry? Using the acronyms above this would be: R - D - A.

I don't think this would cause problems assuming the analog regulator lines are routed through the perimeter of the PCB to avoid the digital circuitry. Mixed signal chips would be placed in between the two regions with the digital and analog sides of each chip facing the corresponding direction. Is this true? Or will I still run into noise issues?

EDIT: it looks like the guide I linked to already answers this question:

Therefore, if the analog and digital return currents (or any two currents) share a portion of the ground plane (i.e., their currents flow through the same metal) there will be crosstalk between the two as the copper resistance causes IR voltage drops.

This indicates that both approaches (R - A - D and R - D - A) are faulty and the circuitry must be laid out in parallel rather than sequentially to avoid this issue.

I will submit this as an answer unless someone has a better explanation.

Answer 1

1. Lets look at this from the signal input direction. The correct order is A-D-R. If you are using a ADC, it would be the final analog device after all of your op-amps, etc. After the ADC all input signals are/should be digital.

2. The proper board layout that I use is that analog ground is isolated from digital ground. They can meet at the ADC IC or at the power source. If no ADC you still merge analog and digital grounds at the power source.

3. If they share the same voltage feeds (+/-5 volt would be typical), be sure to install 100uH radial lead inductors and 4.7uF 25 volt ceramic capacitors at the point where the power is used-not at the power source.

4. The power source has the power connector(s), and the big 470uF to 1,000uF capacitors on each voltage line to ground. This is where ground radiates out in a star pattern to the analog and digital sections. Digital and analog outputs should be close to the power source, as they have loads that often draw much more current than the inputs.

5. As for RF IC's, if used, layout the board as the manufacture specifies.

6. Two simple Golden rules to follow: Never have a digital device use an analog ground trace for a return path. All input and output grounds start at the power connectors.