Connecting Analog and Digital Grounds
The best way is to minimize R &amp; L of current carrying paths for each and ensure path layout does not share ground or Vcc. This includes broad spectrum and DC.  This requires understanding of LdI/dt of pulse noise and stray coupling into high Z inputs by mismatched Z (sig and ground), radiated current loops and E field pF coupling of noise to signals.

If connections are lower impedance than source shared load regulation conducted noise is possible. Then low Q , LC decoupling is necessary with Agnd plane balanced inputs and small loop noise currents using Cap per IC etc.

I wonder still uses extremely thin layers between power ground planes to Z(f) of power planes. There is a business doing this specialty that started around 30 yrs ago.

Additional references:


http://www.ti.com/lit/an/scaa048/scaa048.pdf 
http://www.analog.com/media/en/training-seminars/tutorials/MT-101.pdf
https://www.xilinx.com/support/documentation/user_guides/ug483_7Series_PCB.pdf
http://www.ewh.ieee.org/r4/se_michigan/emcs/DL-ARCH-decoupling3.pdf
https://www.xilinx.com/support/documentation/application_notes/xapp623.pdf
http://ds.murata.co.jp/software/simsurfing/en-us/#app=71e3&amp;ee37-selectedIndex=0


When looking at s21 crosstalk or s22 load regulation, we expect to have a  low source Z(f) and  load Z(f) &lt;1% at DC but AC ratio can be quite different due to a complex distributed RLC network of signals. So an RF Network Analyzer is an extremely valuable learning or debugging tool. 


  The easy way to understand these curves for scattering (s) parameters is that Ohm's Law on a voltage divider is the same as load regulated  ratio of load to source Z(f) ratio in dB. Even dynamic crosstalk and noise ingress can be seen as a transfer function.


Every application may be different and require some of the solutions avail to reduce emanations or susceptibility or both


CM chokes (LF or RF)
balanced differential lines
active guarding on shield or gnd between all A and D signals.
filtering to reduce noise bandwidth
high CMMR before amplification.
strict care on Vref shared grounds to ensure no digital current is shared.