I have read many excellent posts here and a whole heap of other docs but I am still going around in circles.
My question is regarding sharing digital signals between ICs that are supplied by different voltage regulators.
In the first case, I have a PCB that I have partitioned into analog and digital sections.
(Please see attached pdf with block diagram!)
The ADCs are controlled on SPI by an xmega with lots of nice UART ports.
I could connect all devices to the same voltage regulator ("Version 1", top), where I am assuming a star connection is the best.
Alternately, I could regulate each supply voltage separately, ("Version 2", bottom). Here,the different ground planes are separated to prevent disturbances from one voltage regulator getting into another ground plane.
Can anyone comment on the relative merits of the two potential designs?
Is there an acceptable method of routing digital signals between devices with different voltage regulators (preferably without optocouplers)?
In version 2, I have routed the SPI lines, on the back side of the board, through the 'star' connection of the ground planes. Is this sufficient to prevent disturbances, from one ground plane, entering the other ground plane via the SPI lines?
In version 2, is there likely to be a significant difference in the '0V' reference of the SPI lines?
Would be grateful for some words of wisdom!