Phil, et al;
The thing about digital that makes is so bad is that, somewhat like analog, it is a piece-wise/step modulation of the power supply voltage/currents. That said, or understood, what you can think of is a FFT or mixing of frequencies that make up this intermingled with the analog signal. The result can be broadband noise; but, often includes predominate frequencies, as a characteristic of the system.
A method of addressing these is to provide a frequency dependent path to ground. However, this often traverses some impedance, and is thereby limited in it's effectiveness. Another method is to intentionally provide some impedance that prevents mixing. Trouble here is one need know the frequencies in question. Another method is is to combine the forgoing two methods.
The big thing most people over look and that is part of the foregoing is the relative proximity of the digital device and said analog circuitry. Recall I said the signal must traverse some impedance. It's gotta come from somewhere.
Now here's where the wheel comes off for most. Most ignore copper and core losses.
That should point you in the right direction and fuel some further discussion.
