Analogue GND should have been used from the beginning but I left it out by mistake. This omission lead to noisier temperature readings on the expansion board than on the Duet (as documented, with a fix, by David Crocker). This was annoying but I did not see a drop in performance as the thermal mass of the hotends was enough to cancel out any temperature swings commanded by this noise. None the less it needed to be fixed, but in a way that allowed the Duex4s to still be compatible with the Duet 0.6 expansion header.
In the schematic you can see that pin 39 of the expansion header how connects to a jumper, and then on to VSSA (analogue GND) within the expansion board.
AD 12 used to be on pin 39 however it will be used later Duet versions for the probe input on a header on the main duet board.
This allows for analogue GND to be fed in via pin 2 of the header on a Duet v0.6 or for a jumper to be used on later duet boards.The pictures below show the Duet v0.6 and Duex4 v0.2a with the analogue GND fly lead connected to the heated bed thermistor GND.
This fly lead can also be connected to the hotend thermistor ground screw terminal:
or VSSA Pin 38 on the 40 pin motor loom header:
All V0.2a Duex4s will be supplied with the necessary fly-lead for hooking up the analogue GND as described above.