Analog ground, multiple grounds and their meaning - Page 1
Forum section: electronics / beginners

author: Moriambar
author_rank: Unknown
timestamp: November 10, 2021, 10:39:10 pm
content: Hi,this topic has been puzzling me for a while. I mean the fact that we have a distinct ground (analog vs digital for example).I mean: I undersand that "ground" is just a name for "reference potential" and that's fine, but if some systems have to talk to each other they should share a common ref (usually gnd, I think). So what is the purpose of analog gnd vs multiple grounds if (and when) they're all tied together? (If they're not I get it).The only thing I can think about is to have a different separate path for the electrons/currents from/to the power supply. But what is the advantage of this?I hope I made my question clear.thanks!

author: Jwillis
author_rank: Super Contributor
timestamp: November 10, 2021, 11:52:15 pm
content: One main reason you'll see multiple grounds , digital and analog . is so one application won't effect the other . If both the analog and digital are referenced together any sudden change in the analog circuit and can have an adverse effect on the logic reference. These changes can also include line noise . Analog circuits can generally handle a degree of line noise where digital circuits are far more sensitive .

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 11, 2021, 03:33:02 am
content: You have actually explained the reasons for multiple grounds quite well in your question.Just a little more explanation will make it clear. As you say, ground is a conceptual idea - the reference from which voltages are measured. But in reality all circuits are connected to this conceptual point via low value resistors. These resistors are the wires connecting various circuit points to the nominal ground point. The resistances are small, typically a few hundredths of an ohm for direct solder connections up to as much as tenths of an ohm for paths which go through connectors, switches and/or long wires.Now think of a 12 bit A to D converter which will resolve roughly a millivolt in a five volt system. If current changes by a hundred milliamps in the servo section due to motor start up, or in the supply due to turning on a heater even resistances on the low end of the range for the system grounds will cause LSB changes in the converter. Those current changes are one to two orders of magnitude smaller than might be observed in many real systems. Audio systems were the desired noise levels are 50 to 70 dB below the saturation level result in similar exposure to stray current induced voltages as the A to D case.Star grounding systems control this problem by providing separate paths for current changes in the higher current parts of the system, leaving only tiny currents and thus voltages relative to the nominal grounding point in the leg served by sensitive (analog) portions of the circuit. Similar logic isolates digital signals which have large amplitude signal swings, even though they are usually not high current. But these digital signals often change at high frequency where not only the DC resistance but the AC impedance matters. It is difficult to reduce the AC impedance to extremely low values.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: November 11, 2021, 04:32:21 pm
content: If you have a very sensitive oscillator and want 2 in your project, you need 2 ground lines, else they retrigger eachother.Your grounds should be tied together after the PSU-input capacitor.Funny : You could make a switch so the ground will be connected close between the oscillators, locked oscillators : on/off,it does something, its looking like it would do nothing.

author: Ian.M
author_rank: Super Contributor
timestamp: November 11, 2021, 04:54:40 pm
content: Shades of 'More Magic'!http://www.catb.org/jargon/html/magic-story.html

author: Moriambar
author_rank: Unknown
timestamp: November 11, 2021, 07:08:01 pm
content: Quote from: Jan Audio on November 11, 2021, 04:32:21 pmI'm sorry but I don't understand why any of this could happen, nor why they should connect after the capacitor, not even actually where in the circuit would be "after". Would you care to explain, plz?

author: Moriambar
author_rank: Unknown
timestamp: November 11, 2021, 07:09:15 pm
content: Quote from: CatalinaWOW on November 11, 2021, 03:33:02 amah, ok, so the general idea of electron paths was kind of ok. Thanks for your in-depth explanation

author: Siwastaja
author_rank: Super Contributor
timestamp: November 11, 2021, 07:33:33 pm
content: Yes, just remember that with high-frequency AC part of signals (some MHz; not audio, audio is nearly DC in this regard), return currents automagically follow the ground plane under the trace, so adding cutouts and multiple planes usually only makes things worse (by forcing the return currents to take a longer loop, radiating emissions, if you add cutouts in wrong places, that is). In other words, DC return currents take theshortestpath, AC takes thesamepath as the signal itself; hence a plane works for AC to minimize the loop, but with DC, you get ground plane voltage disturbances even outside the track because return current takes a shortcut elsewhere. As a result, routed ground tracks (or segmented planes) may offer some benefits for DC.(This is of course a simplification, really current doesn't take any single path but follows some distribution which you could simulate with expensive EM field simulators. But you can approximate it as I said.)Rule of thumb is, all those multiple ground planes / plane cutouts tricks are only used forhigh accuracy, low frequencythings. For example, when any DC offset on a 16-bit ADC conversion matters.From EMI perspective, full solid ground plane is almost always the best way to do, and it obviously happens that such plane is quite good for DC resistance, too.But sometimes this "quite good" isn't enough, e.g. when single design mixes high currents and high accuracy small measurement signals. An example would be a 50A power converter with 16-bit current/voltage measurements. In such instruments, ground plane cutoffs and similar trickery are needed.Easiest, but also quite expensive way to sidestep the whole issue is to create completely isolated (both power and communication) modules.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 12, 2021, 03:57:51 pm
content: If you have an IC with digital and anlog grounds, you tie them together as close as possible. You have distinct subsystems, then bring dgnd and agnd back to a single ground point immedaitely adjacent to the power supply or power supplies.Why, well there is (generally) much more crap on the dgnd line than the agnd line. System shielding gnds connections are tied to the gnd of the circuitry being sheilded. Then there is also "guarding" in grounding schemes- read up! Falsely people assume all gounds are equal and are at 0.000000... V which they are not. There is a finite impedance to the line , so any current flowing (AC or DC) causes PDs to arrise shifting gnd away from 0V. Seehttps://www.autodesk.com/products/eagle/blog/8-pcb-grounding-rules/Note that this impdance is not purely resistive. It has indcutance and capacitance. The pupose of bypasing caps is to try and mitigate these effects.Neglecting grounding and power schemes is the no 1 cause of unexpected behavior in your project. The higher the frequency- the worse it getsMains ground is another subject. Sometimes the terms gnd and earth gnd are used interchangeably. They may be tied together depending on the situation. They often are. Earth gnd is mostly used a path for fault currents (to take out your breakers and fuses) its not all reliable as a 0V referrence point. My earth is not your earth- possibly by thousands of volts, this is what often takes down power grids.Grounding and shielding is a complex subject in its own right.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: November 12, 2021, 04:24:16 pm
content: Quote from: Moriambar on November 11, 2021, 07:08:01 pmYour power goes into to capacitor, then behind that, as first.