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I am building a system with multiple PCB cards.

  • The whole system has a Chassis Ground; connected to a main Power Supply reference.

  • All cards will be powered in reference to Chassis Ground.

  • Each card has its own Isolated supplies ie DCDC for Digital Signals & DCDC for Analog Signals

  • Data will be transferred along IO lanes in reference to Digital Ground

Basically this diagram:

enter image description here

Now the problem I have is how to reference the Digital IOs of each card to the Mother board's digital ground? If I were to just connected the reference of each cards digital supply I assume I would have substantial amounts of current flowing between the cards and the digital reference would be constantly moving around.

Or have both the digital reference and supply provided by the mother board, but I want this design to be expandable and not have to worry about increased current demands if I add more cards.

Ideally I want the cards to be completely isolated. Some of the things I thought of were:

  • using back2back schottky diodes to reference each boards digital ground to the mother boards. Though this could cause IO levels to be out of spec and damage the electronics.
  • using an OPAMP to create a ground reference on each card. I thought maybe the opamp would then start sinking all the circuits current rather than DGND, but if the OPAMP is on different supplies I think thats impossible.
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    \$\begingroup\$ I don't get it. Imagine a computer with many ISA or PCI slots, what's wrong with that. And why/how the current would flow from card to card? \$\endgroup\$ – Marko Buršič Jan 18 '18 at 13:03
  • \$\begingroup\$ I want the cards to be completely isolated there should be a good reason for doing it as it creates electrical and EMI complexities. Please explain why this requirement takes place. \$\endgroup\$ – Anonymous Jan 18 '18 at 13:04
  • \$\begingroup\$ Also it makes nonsense to use isolated DC/DC converter and then use the same GND for data comm. If you want to isolate then use a barrier for data, also. \$\endgroup\$ – Marko Buršič Jan 18 '18 at 13:08
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Use a barrier - optocouplers, magnetic couplers, transformers,...etc

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Let me clarify, I need to have each card's Analog GND, Digital GND, and Supply GND all Isolated. Though the Digital GND does not necessarily need to be Isolated from card to card. I just thought It would be Ideal for whatever reason. I just looked up digital isolators... I have 64 IOs, not worth it. If I can get away with just using a Digital Common that all the Digital Supplies will be referenced to without causing the reference voltage to be unstable Ill do it that way. \$\endgroup\$ – Tony Jan 18 '18 at 13:50
  • \$\begingroup\$ Actually what you try to implement is a PLC. There are many ways to do it. You can use a MCU and then send just the status via some communication bus, then you will need only few isolated channels. The other way is to use opto isolators on each dig. input, that's the common way to do it, because you don't want to share the cpu gnd with external devices. \$\endgroup\$ – Marko Buršič Jan 18 '18 at 13:57

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