The basic idea to avoid high-frequency interference is to minimize the interference of the high-frequency signal electromagnetic field, that is, the so-called Crosstalk. You can increase the distance between the high-speed signal and the analog signal, or add ground guard / shunt traces next to the analog signal to avoid high-frequency interference. Also, don't foget to pay attention to the digital ground to the analog ground noise interference.
A multi-layer printed circuit board is constructed to suppress radio frequency interference (RFI) generated by high frequency clock and data signals therein. Suppression is achieved by positioning clock lines carrying the clock signal on a first voltage reference layer proximate to a second voltage reference layer. The two layers shield the clock signal from the signal lines on other layers. Noise may be further reduced by forming bridges in the second voltage reference layer proximate to the clock lines so that the bridges span the width of the clock line. Capacitors are also preferably utilized to further suppress radiated noise. The capacitors provide AC coupling between the first and second voltage reference layers so that a low impedance path is provided for high frequency noise generated by the clock signal. The second voltage reference layer thus operates as an effectively continuous shield between the bridges.
