Multicard System Grounding

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:

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.