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In my design I have four different ground:

Ground: from USB for step up and battery charger IC.

Ground1: for a split supply (virtual ground).

Ground2: for the input signal reference.

Ground3: for the output signal reference.

how do I physical connect them together (if I have to) in the PCB layout?

enter image description here

enter image description here

Status Re
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  • Is it acceptable for these four nets to be isopotential? – crasic Nov 08 '21 at 16:58
  • The virtual ground cannot be connected to the other since it is (probably, assuming I understand what you're doing) a different voltage. The other should (probably) be shorted through a large ground plane. But show a schematic for what you're building, otherwise these are just guesses. – user1850479 Nov 08 '21 at 17:01
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    Put some time to understand how and where the current flows in your circuitry first, and the potential references. Eventually, connect all grounds together, in the way... current loops do not cross each other. The name "ground" is always confusing. Try to think it as "reference point of potential differences" – jay Nov 08 '21 at 17:01
  • I have added the schematics – Status Re Nov 08 '21 at 17:10
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    If there is more to the schematics than what presented above, show it too and please explain what the circuit does. But it is a red flag to see USB input connected as GND but then later there is another net called GND which is not at the same potential and is likely the analog ground. There is a good chance that if your device has other external connections such as audio, someone connects both the USB and another connector to same device, shorting the battery, melting wires, destroying the connected device etc. So as it is now, connecting grounds together shorts the battery. – Justme Nov 08 '21 at 17:19
  • the upper circuit is a power supply. I have used different symbol for two different ground refences. The lower circuit is an headphone amp with 1 input and two outputs. the ground reference is the one from the split supply. Plus I have input reference and output reference. – Status Re Nov 08 '21 at 17:25
  • It doesn't matter what symbol you choose. If the net name is GND, the Pcb editor will treat it as the same net. It is not the same net as @Justme explained. – tobalt Nov 08 '21 at 19:26
  • The GND connections of the two LDOs and that of the virtual ground (from batteries) should be named gnd1 too. I have just realized that i posted a wrong/old schematic. I was keeping gnd and gnd 1 separated from each other – Status Re Nov 25 '21 at 07:48

2 Answers2

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how do I physical connect them together (if I have to) in the PCB layout?

You can't, because:

  • if the USB charging port is connected to a PC, then the charger ground is connected to the PC ground.

  • if the audio input jack is connected to the PC's audio output, then its ground is also connected to the PC ground.

enter image description here

The result is that one of the lithium batteries gets shorted through the cables' grounds. This will most likely melt some of the connectors and/or set the battery on fire.

To solve this problem, you can:

  • Use a DC-DC converter with a positive and negative output to charge both batteries, which then makes the USB charging port ground and the audio jack grounds the same

  • Or use a canned isolated dc/dc converter to break the loop.

Also note the INA differential receiver is not serving a purpose since the input signal is not differential. It is just an expensive buffer.

bobflux
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  • All right. I will need a transformer driver and a transformer before the boost converter. Or is there a 1:2 smd transformer for at least 350mA? And instead of the INA what about a NE5532? – Status Re Nov 09 '21 at 07:02
  • Look into the ["isolated DC-DC converter" category](https://www.mouser.com/c/power/dc-dc-converters/isolated-dc-dc-converters/) there are plenty of choices... NE5532 wouldn't work because it isn't rail to rail, its input common mode has to stay a few volts away from the power supplies, so it's not suited to low voltage designs... Just checked your INA and maximum input voltage is also VCC-2V so that would be pretty tight too. You'd need a rail to rail opamp. – bobflux Nov 09 '21 at 08:38
  • But you could also use only one LiIon battery with charger, and use a DC-DC to make +/- voltages for the opamps from the battery too, that would allow higher supply voltage and output voltage for your headphones if that's needed. – bobflux Nov 09 '21 at 08:42
  • Of course using only one cell would make things easier , but I couldn't find a DC-DC with enough current for the lm 4880s. I keep on forgetting to check for rail to rail operation :( – Status Re Nov 09 '21 at 09:51
  • Is this for a 8R speaker or headphones? – bobflux Nov 09 '21 at 09:59
  • Just for Headphones – Status Re Nov 09 '21 at 10:42
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An experienced PCB designer will certainly use one layer per GND signal and will connect all these layers to the GND pin of the connector of the board (common return point for GND currents).

Since money always count and you probably have just one layer you have 2 approaches here:

  1. Connect all your GND signals to the GND Layer using vias.

  2. Guess which GND signals switch faster and look around for "GND layer split in PCB routing".

Enrico Migliore
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    This question is only in part the usual "should I split my plane?" type question. There GND nodes do have different potential, so cant be all joined. – tobalt Nov 09 '21 at 04:42