Proper way to tie Analog Ground, Digital Ground, and Earth Ground together. - Page 1
Forum section: electronics / projects

author: SirDan
author_rank: Contributor
timestamp: April 22, 2019, 11:32:46 pm
content: Hello,I have been searching the net for answers on how to properly tie Analog Ground, Digital Ground, and Earth Ground together on our PCB. I found many ways to do this. Some use caps, some use resistors and caps, etc. I found a forum post onelectronics.stackexchange.comand chose a variation of the method the top answer likes the bestQuote.The reference design that our project is based on shows the grounds tied together like the attached image, through a bead and a resistor: reference-design-grounds.jpgI marked L1 and R1 do not populate on the schematics so I have some options when EMC testing.What I have done is shown (attached) in layout.jpgJust looking for any opinions on this. What is the best way? Does the layout we have look OK?Regards,Dan

author: OM222O
author_rank: Frequent Contributor
timestamp: April 22, 2019, 11:53:05 pm
content: you need to use the star grounding technique. 0ohm resistors or inductors are used in order to allow the PCB design software to distinguish between the nets and to avoid ground loops. for prototyping on a breadboard or similar, just use 3 wires tied at one end to the common Earth/AGround/DGround and use each of the other ends for a seperate circuit.

author: DaJMasta
author_rank: Super Contributor
timestamp: April 23, 2019, 01:13:30 am
content: Yep, star grounding is the right approach. Putting extra impedance between the local and common grounds means you can end up with different potentials on each side, much moreso than is generated by a nice large grounding trace or wire. If your grounds are at different potentials, that's where you run into trouble, so a low impedance path that meets at a primary point keeps the analog and digital grounds at similar potentials, while still keeping them somewhat separate, and prevents one of your eventually connected grounds from moving to something else and causing trouble.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2019, 05:47:05 pm
content: If you don't know what you're doing, tie everything together, as often as possible, as widely as possible -- on a ground plane.When something is looking to need some ground isolation, don't try and hack it. Isolate it fully, and deal with signals and the common mode as you can. If management / customer balks, just remind them of what they are truly asking for. They may decide to relax the spec as a result, or they will proceed with a higher quality design than the compromise they had anticipated.It takes years of theoretical and practical knowledge, perhaps decades, to understand all the factors in play here. Even for all my experience, I don't have much confidence in pulling off anything fancier, and even then only in extreme cases that are worth the extra consideration.I cannot begin to answer your particular question from the information provided. To do that, I would require as inputs, what all the connecting cables are intended to be connected to, what they may (unintended) be connected to, what the enclosure and connectors look like, and what the complete circuit looks like, both in schematic and layout. (I don't necessarily need the whole circuit, but the kind of information that I'm looking for is again hard to describe. The circuit may not've been designed or laid out in the same format as the information I need, so it's most likely easier to read the design myself, and parse it into the format I need.) Finally, I need the EMC specs (if any) for the project.I hope that, by enumerating how much is required to make a judgement, that this gives some perspective on how in-depth this question really is.Tim

author: capt bullshot
author_rank: Super Contributor
timestamp: April 23, 2019, 06:37:55 pm
content: Quote from: T3sl4co1l on April 23, 2019, 05:47:05 pmI'd expand that to "if you don't know or if you think you know what you're doing". Otherwise nothing to add.There's a lot of use cases that may require isolation here. Most of these cases require "real" isolation, like T3sl4co1l said. Only a few may work without, but most approaches I've seen to separate Earth and circuit GND without real isolation failed in some way.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 24, 2019, 02:24:32 am
content: On a related subject, a recent project I've been working on, started out with half a dozen groundson one board alone!Worse still, it was two layer routed, no planes/pours; and worst of all, it was just a switching power supply, no reason at all to split grounds! Needless to say, it was an EMI nightmare beyond a few MHz. I talked them down to a 4-layer ground plane design with only three ground domains. Connections between domains are made either with common mode chokes, when the connections are between galvanically linked domains; or with isolators and optional 'Y' capacitors, for galvanically-isolated domains. The results will be coming in soon, hopefully with significant reduction in emissions.I told them, that one board had more grounds than any project I had worked on up to that point,including multi-board designs linked by cables, counting each board's local ground separately.That fact didn't seem to impress them very much though...Quote from: capt bullshot on April 23, 2019, 06:37:55 pmYes, quite; to omit the Dunning-Kreuger effect would be negligent.There is something very dangerous, I think, about discussing alternate grounding designs at all. It's an important design tool, on the rare occasion that it turns out to be needed; but the number of articles discussing it, is absolutely blown out of proportion to the number of applications that actually need it.So, reading on these topics, can give the novice the impression that these things are quirky and cool and useful, far more often than they are -- without understanding the huge scope required to make such a judgement. Most especially, those that "know just enough to be dangerous", or that think they know everything that needs to be considered.Life is weird, on the far side of the D-K curve. I can't honestly give an answer with certitude, because I know not only thateverythingis uncertain, but that my knowledge is itself uncertain, to varying degrees. Mind, the uncertainty associated with core, well-practiced topics (like circuit theory, or E&M), is very small indeed. But it would be foolish of me to state otherwise, especially when the information feeding those tools is itself uncertain. And equally so, topics are around the edge of my knowledge are much less certain, and consciously so. (Mind, there is the know-it-all's disclaimer: I can infer or extrapolate on those topics, and sound awfully scientific and certain about it in the process, but really I'm just grasping at straws, constructing hypotheses about it -- which may well be obviously wrong to someone with a deeper knowledge of those subjects!)The other grounding topics that come to mind, for those curious -- slotted grounds, trace routing (path of least impedance), star grounding*, and similarly for supplies: local (usually analog) supplies, gaps between planes, etc. Shielding of cables (protip: always ground both ends, to do otherwise is approximately equivalent to not shielding at all). Ground loop (especially in audio and metrology).*There are two kinds of star grounding, in fact. One is the somewhat naive kind, which is done to minimize errors in precision metrology instrumentation -- it's only applicable to low frequencies, where the inductance of those return paths can be ignored. It falls over, in many magnificent ways, at high frequencies. Which may be mere kHz for precision AC, or low MHz for outright instability in the circuit.The other kind is nuanced by the consideration of fields -- namely, following the path of least impedance back from each subcircuit, so that signals between subcircuits pick up as little error as possible due to induced voltage drops on each "spoke". Here, the topology resembles a central hub with spokes. All power, signals and ground are routed from each subcircuit back to the hub. I suppose "hub" is a better fit than "star". The important part is, signals get shielded by their grounds, so they do not pick up induced noise between subcircuits, whereas in a naive star-ground design, signals may end up routed between subcircuits by straight line connections, where they pick up all the awful induced voltage between spokes. From another perspective: it violates each ground domain by routing a trace away from local ground; this is why you must never route traces over a ground split/slot....I hope I'm not lecturing too much and that it comes off as self-inflating. I mean, I do like talking about high level topics, but I want others to learn these things. The world needs more, and better, engineers! I suppose a lot of "thinking about thinking" can seem redundant, or confusing? Critical thinking is key, though. Once you can critique anything, and learn about anything, the world is yours.Tim

author: iMo
author_rank: Super Contributor
timestamp: April 24, 2019, 12:41:42 pm
content: https://www.maximintegrated.com/en/app-notes/index.mvp/id/5450

author: David Hess
author_rank: Super Contributor
timestamp: April 24, 2019, 01:21:26 pm
content: Quote from: T3sl4co1l on April 23, 2019, 05:47:05 pmI do not find that surprising. The theory and practice are mostly invalid because the simplified models used do not reflect reality. So the usual fallback position is to use a solid ground plane and decouple everywhere and even that completely fails to meet the requirements of some circuits.Offhand I do not remember any academic publications which got it right at a practical level. Analog Devices publishes several application notes which do though and I suspect that is because mixed-signal design is so demanding.Use a single point main ground, and then draw out all of the return current paths to identify where islands and isolation are required.