Yes, it's DGND. Once I connected DGND and AGND on the schematic, the PCB design program treated both the same in terms of the rat's nest so it would be really easy to screw up the layout, at least in terms of preserving the desired "star" connection. I _think_ I got it right.
Hi Mark
Yes indeed, DGND pin is a very special sort of digital ground, and it does get connected to GND at the ground plane right under the chip. The state of the SAR switched capacitor array is transferred into a register in a low energy manner, and the internal star point is GND, connected to DGND.
DGND is noise free, returning currents from DVDD, and does not share significant energy with OGND.
The interface digital circuits are powered from OVDD, returning to OGND. These two are separately provided for the purpose of reading out the result to eternal digital devices without any of the associated currents becoming involved with the ADC measurement front end.
Speaking of layout issues, I was considering separating OGND from DGND but due to the way the ISR runs the ADC, either the digital section is running or the output section is running: but not both at the same time. So it seems to me that it should be OK for them to share the same ground.
DGND and OGND are deliberately provided separately, for exactly the reason that they never do (share the same ground). The whole idea is not to pollute the sensitive capacitor SAR array, and its comparator, with any clocking, digital data, and other currents associated with the data interface, and OGND. One should not casually decide to subvert this aspect of the ADC design. If it was OK to join OGND to DGND, it would have been done in the chip.
This is why I made the supplies be independent, and isolated. Yes, I derive the analog supply from the 3.3V, and I use a LDO regulator, but the low power high frequency isolation transformer barrier ensures that no currents involved with reading out digital data can ever run in, nor return to GND nor DGND. I once considered it might be easier to use a couple of AA batteries!
If one simply uses the same supply from the computer circuits, regulated down as necessary, and hope to supply the ADC from it, then necessarily, the GND, DGND and OGND all end up connected together somewhere. This bypasses the design arrangement that provides the separate OGND for digital interface transfers.
Nor can it be about when one or the other is not using it simultaneously. If the computer ground finds it's way to the analog GND, then it, and it's noise, and the voltage developed from the return current across the ground plane, and 0V wire going to the computer, will be there, regardless of when you happen to be transferring data.
