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.

  • \$\begingroup\$ It would help us if you show a simple diagram of the PCB you are designing - at least to show how you are positioning the analog and digital sections within the PCB's outline and the basic dimensions. Also, what is your skill level with PCB layout? (e.g. beginner, experienced, very skilled). \$\endgroup\$
    – FiddyOhm
    Jul 26, 2016 at 21:10
  • 2
    \$\begingroup\$ Just read that app note, and keep re-reading it. Anybody that posts an answer will do little to better it. Especially re-read the paragraph 'ground is not an equipotential' a few times more. If you try to boil it down to the simplistic 'what order?' of your question, you will be almost certain to run into problems at some time. \$\endgroup\$
    – Neil_UK
    Jul 26, 2016 at 21:32
  • \$\begingroup\$ Yes, I did a word search for "analog" and found that the "Ground is not an equipotential" section answered my question. I've noted this in my post, let me know if you see any problems. \$\endgroup\$
    – SharpHawk
    Jul 26, 2016 at 21:36
  • \$\begingroup\$ You use the phrase "analog regulator lines", does this indicate your PCB has both analog voltage regulators and separate digital regulators? \$\endgroup\$
    – FiddyOhm
    Jul 26, 2016 at 21:56
  • \$\begingroup\$ @FiddyOhm: Correct, there are separate regulators for analog and digital devices. The digital regulators power digital devices and the digital Vcc's of mixed signal chips, and likewise with the analog regulators. \$\endgroup\$
    – SharpHawk
    Jul 26, 2016 at 22:11

1 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.

  • \$\begingroup\$ So how does this A-D-R scheme fit into the OP's situation where he has two separate regulators (analog & digital). Are you saying the analog ground should connect to the digital ground, then this combined ground goes to the regulators? Or should it be Ra-A-D-Rd? \$\endgroup\$
    – FiddyOhm
    Jul 26, 2016 at 22:33
  • \$\begingroup\$ Yes. You can only have one 'original' ground, so it is difficult to just use the abbreviation R-D-A. If branching out it is more like RR (all regulators/pwr feeds)-DD (multiple digital logic)-AA (multiple analog devices). The key is NOT to have any analog grounds contact a digital ground except at the power source. The same could be said for power feeds. \$\endgroup\$
    – user105652
    Jul 26, 2016 at 23:19
  • \$\begingroup\$ I agree as far as IR coupling is concerned. \$\endgroup\$
    – FiddyOhm
    Jul 26, 2016 at 23:28
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    \$\begingroup\$ Do you agree with the layout shown in figure 27 of the Maxim guide? Mixed signals chips would be placed in between the digital and analog circuitry and oriented accordingly, of course. \$\endgroup\$
    – SharpHawk
    Jul 27, 2016 at 15:58
  • \$\begingroup\$ @SharpHawk. Yes, it is better than figure 26. Though it is not show I would keep all analog grounds together in a star pattern, then a trace from the star pattern connects to the digital power source. Figure 27 and others imply that analog is being fed to digital section for ADC conversion. If figure 27 implies that 'pristine' analog ground can be mixed with a noisy digital ground, then that drawing is wrong. \$\endgroup\$
    – user105652
    Jul 27, 2016 at 18:00

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