I want to make a board that have differents ICs and looking at the datasheets, I can see different grounds connected in different ways and I'm not so sure about how to connect them/which ground should be ground plane (earth,AGND,PGND,SGND,GND)/should I make different ground planes/etc.

I use KiCad 7.0

my components are :

  • MAX17571ATC+ to convert 24V to 5V
  • MAX3098EACEE+ for reading an encoder
  • ISO1I811TXUMA1 so I can have 24V inputs
  • ISO1H816G so I can have 24V outputs

the ISO1I811TXUMA1,ISO1H816G and MAX17571ATC+ have evaluation boards with schematics so that helps.

For example, in the evaluation board of the MAX17571ATC+, GND and earth are connected together, but in the EVAL-ISO1H816G board schematic, there's a 4.7nF capacitor between Earth and GNDbb1.

from datasheet

from datasheet

Also, what's the difference between GNDbb1 and GNDcc? are they the same? if not, which GND is going where?

thanks for your help.


1 Answer 1


The 2x8-ISO1H816G-1 evaluation board has separate grounds for the CPU-side and the switched-side -- it's an isolated switch. The eval board has two of the 8-output chips and so has two isolated grounds GNDbb1 and GNDbb2. If your 16 things share a common ground you could connect these two together.

The 2x8_ISO1I811T eval board is the same in reverse, but the chip's GNDbb is labelled as PGND in the circuit diagram. If your 24V input things are shared with the outputs, you could connect this to the GNDbbx of your outputs.

The MAX17571 eval kit eval kit has the signal ground SGND and power ground PGND directly connected. If this is for powering a microcontroller which is connected to the digital side of the isolation boards, then this is connected to GNDcc of those boards.

You've got isolated input and output boards, but it looks like you're wanting to power the 24V devices from the same supply as the 24-to-5V converter.

  • If you want the isolation, you need a separate 24V power supply for that side. +24 and GND of that supply go to Vbb and GNDbb
  • If you don't want the isolation, why are you using isolated input and outputs?

The classical way for an isolated system is like this: enter image description here

If you replace the mains-5V supply with a 24V-5V supply, fed from the orange, you'll cross the green line and break the isolation.

(The reason the mains is allowed across the green line is because there is isolation in the transformers of the mains-DC supplies, and you can think of the green line as going through the transformers. There's a good summary at Galvanic Isolation in Wikipedia.)


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