Distributing Ground and Clock Between ADC and FPGA on Separate PCBs
Using isolators is the most straight forward way to couple boards like this while avoiding grounding and noise headaches.

For an isolated design to function, you need isolating buffers for all the digital input and outputs as well as an isolated power supply, although the later can be ignored if the power ground acts as the "star point" for your daughter board and you do not require isolation on the sensor part (e.g. a 'floating adc').

Conceptually this can be imagined by the following diagram







simulate this circuit &ndash; Schematic created using CircuitLab