you do not need to ask permission to ask questions - just ask
when you say substrate, do you mean your designing an integrated circuit, or do you mean a printed wiring board?
unless i completely misunderstand, when you say bias current, i assume you mean local power for the circuit, Vdd and Vss.
the digital and analog Vdd and Vss should be separated
somewhere along the line, there must be some sort of power input to your board. the grounds get tied together there.
use a big and fat a trace (copper pour / ground plane) as you can - this keeps the current density down and any radiated EM down.
you might want to move all of your bias current generation to a third section with its own ground plane.
again, tie them all together at one point.