Go to TI's web and look for Practical Analog Design Seminar under the Technical Documents section. Chapter 3, signal integrity, contains a discussion on generated noise and decoupling that you should read. If you want an answer without knowledge put two .1UF ceramic caps on each digital IC, and filter the power lines to the analog circuits with RC filters comprised of 51 ohms from Vcc in series with .1UF ceramic cap to ground, and OP amp power taken from the junction of the RC..