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[cli21]

I have a question about a schematic of evaluation board provided by a company.

The lower right corner of the schematic below shows a simple circuit for the power supply section of the evaluation board. It seems that there are two ground, one is GND and the other is FG, there can be connected together through a jumper. The components on the board are connected to GND rather than FG.


This kind of design made me confused.
I think in this case, most area of board(copper pour) will be FG, GND only exists in several traces, am I correct? If I am correct, then what's the benefit of doing this?

[uChip]

Without seeing the entire circuit diagram it is really difficult to say for sure, but I will guess. It probably has to do with noise isolation and suppression. FG may stand for filtered ground which is connected to ground someplace besides the jumper through some lowpass isolation filter. The few traces not connected through FG are the connections to the filter and perhaps some noise insensitive grounds that require a lower impedance.

Again, without seeing the entire circuit my idea is really just a wild guess.

- Chip
I do not work for SparkFun.

[andyFraser]

I think you will find that FG stands for frame ground and will be connected to the pads that form the mounting holes.
For EMC reasons you will often find that the electronics ground will be connected to the metal enclosure through the mounting holes and by joining the frame ground to the electronics ground through a jumper, this can be done after the unit is assembled.

Andy

[cli21]

Without seeing the entire circuit diagram it is really difficult to say for sure, but I will guess. It probably has to do with noise isolation and suppression. FG may stand for filtered ground which is connected to ground someplace besides the jumper through some lowpass isolation filter. The few traces not connected through FG are the connections to the filter and perhaps some noise insensitive grounds that require a lower impedance.

Again, without seeing the entire circuit my idea is really just a wild guess.

- Chip
I do not work for SparkFun.

Hi, I've re-attached the circuit diagram. Sorry about that. Hope you can check again.

[cli21]

I think you will find that FG stands for frame ground and will be connected to the pads that form the mounting holes.
For EMC reasons you will often find that the electronics ground will be connected to the metal enclosure through the mounting holes and by joining the frame ground to the electronics ground through a jumper, this can be done after the unit is assembled.

Andy

Hi Andy, I've re-attached the circuit diagram. I think what you said is reasonable. In this case, it seems when I design the pcb using EDA software, I should pour copper on top and bottom layers, since there layers are connected to mounting holes, so they are actually the FG. But in the diagram some of the components have a GND pin, like op amp, I should connect the GND pin to the GND on the left of the jumper by routing a single trace, so finally, large area of the board is FG, only some traces are GND, is that correct?

[andyFraser]

It needs to be the other way around. The pads on the mounting holes should be connected to the FG net so the ground pours on both sides of the board don't connect to the mounting pads. All component grounds should be connected to your ground pours which shound be the GND net. I would guess that only one mounting pad is FG and it is the one connected to the jumper.

Hope that makes sense.

Andy

[cli21]

It needs to be the other way around. The pads on the mounting holes should be connected to the FG net so the ground pours on both sides of the board don't connect to the mounting pads. All component grounds should be connected to your ground pours which shound be the GND net. I would guess that only one mounting pad is FG and it is the one connected to the jumper.

Hope that makes sense.

Andy

Hi Andy, if you check the picture of the board, you can see that the GND is restricted to a small area on the lower right corner of the board, this area is isolated from the surrounding large board area. So I think the even if I pour copper on both side, this copper region will not be GND. The components grounds have to be connected to the small restricted GND area by single trace. What do you think?

[andyFraser]

In the small ground pour you can see a number of vias which I suspect connect to a much larger ground pour on another layer (bottom if that is a two layer board).

Andy

[InactiveUser001]

As above, connecting the GND directly to the metal chassis of an enclosure can sometimes cause problems, I can recall in fire alarm boards that we would a link that can be added\removed in case of ground loops causing problems.