I've run into a connection between two PCBs, and it's not giving me a warm fuzzy feeling. The master is a carrier board, and the second board is a standalone sensor that communicates over SPI. This is more of a thought experiment for me to form a better conceptual understanding of EMI issues. I understand there is a lot more at play here, signal rise time, frequency of the SPI communication, proper line impedance and termination, but I want to focus on cable setup.
In my mind, this is a problem because power and ground are no longer tightly coupled, creating a large loop area that is ripe for high-frequency EMI noise. similarly, the SPI bus forms a large loop area and is also susceptible to EMI noise
Without changing the design too much, I would imagine the best option would be to remove power or ground from the SPI cable. The issue of a large loop area still stands, but the noise is localized to the SPI instead of the entire system.
Ideally, I would think bring the power across with the communication lines and using a properly shielded cable would be best.