Confusion regarding different ground symbols and their usage - Page 1
Forum section: electronics / projects

author: Ranayna
author_rank: Super Contributor
timestamp: August 10, 2018, 09:39:32 am
content: Hi everyone,at the moment, I am trying to build a Power Over Ethernet 5V / 4A Powersupply, using the ST PM8803 Chipset.I am still very early in the project, I have just yesterday finished transferring most of the reference schematic into Kicad.Here is the reference document:https://www.st.com/content/ccc/resource/technical/document/application_note/aa/00/34/1b/19/82/4a/da/DM00113368.pdf/files/DM00113368.pdf/jcr:content/translations/en.DM00113368.pdfThe schematics I am talking about are on pages 5 and 6.I am somewhat confused by the usage of ground symbols in the schematics.I see a total of four different symbols used:- A fork-like symbol, labeled "Chassis". It is only used on page 5 and seems obvious: it is the shield of the RJ45 jack. Please note that, unlike the reference, I want to use a jack that has the magnetics integrated. The capacitor C12 is integrated to that jack.- A downward pointing triangle. This is tied to the chassis symbol on page 5, but also extensively used on page 6, but only in the isolated section, if I see this correctly.- A classic three line earth symbol, labeled RTN, used both on page 5 and 6- Another classing three line earth symbol, labeled ARTN, used on page 6On the schematic on page 6, RTN and ARTN are tied together. RTN is coupled to the "triangle ground" with a high voltage capacitor.I am unsure how to handle these 4 different grounds.My first question may sound stupid, but I want to be sure: Am I correct in the assumption that the same symbols in the two different schematics are tied together? Meaning the "triangle" ground on page 5 is the same as on page 6?How much do I actually need to separate RTN and ARTN? I found conflicting information regarding this, concerning digital and analog ground planes. If I need to separate them, where should I connect them? I would do so near the chip, since the RTN and ARTN pins are next to each other.If I see it correctly, the triangle ground will be at 0 volt in regards to the output of the power supply, since it is only used in the isolated section. If I then tie this to the chassis (shield) ground and then use a shielded network cable, it would be at earth ground (via the switch, if it is grounded itself). I must keep this isolated from RTN and ARTN.The capacitor used to couple RTN and triangle ground, while it is not specified as such in the BoM, I would think it is a good idea to use a proper class-y capacitor, or is that not necessary?And, finally, last: my intention is to place a Wiznet W5500 chipset on the same board, fed from a 3.3V regulator. The grounds of that part of the circuit should tie to 0V of the power supply, correct?I hope this is not too much asked at once. For me it definitely is

author: Siwastaja
author_rank: Super Contributor
timestamp: August 10, 2018, 10:03:28 am
content: Everyone uses whatever symbols they feel like using. Using a single symbol with different net names (DGND, AGND, etc.) next to the symbol makes more sense. You don't use separate power pin symbols (bar, arrow, etc.) to denote different voltages, either; you use text to describe which net it connects to.If the document lacks documentation on what whichever symbol means in their nomenclature, then you need to reverse-engineer the intended purpose by experience.Additionally, incompetent design engineers, often tasked with writing application notes (usually summer workers do these!), tend to copy others in creating too many "different grounds". Sometimes when you see four, only two or even just one is needed. Almost 100% of the time, when you see AGND and DGND, only one is actually intended, and using two generates EMC issues.I expect this trend is going to disappear, since now it's becoming a trend that everybody "knows" that the split plane is poor design (and you can find a lot of threads about this!); similarly, ten years ago everybody "knew" itisneeded.

author: Benta
author_rank: Super Contributor
timestamp: August 10, 2018, 11:57:13 am
content: I have to agree, ground symbols are often used indiscriminately and everyone has his/her own drafting style.There is only one well-defined symbol, and that is "Protective Earth", which may only be used for PE and should always be used for PE:

author: Bud
author_rank: Super Contributor
timestamp: August 10, 2018, 01:46:03 pm
content: I'd check with your country's electric Code if it defines specific use for specific ground symbols.