Analog and digital ground plan with isolated DC-DC converter

Question

I use two isolated DC-DC converter in parallel from 24V

1. One TDN 3-2411WISM (single output) to create 5V for the "digital world" followed by LDO to create 3V3 and GND for the VDD (+Vout) and VSS(-Vout) of my microcontroller (STM32).
2. One TDN 5-2422WISM (dual output) to create -12V (-Vout), +12V (+Vout) and AGND (Com.) for the "analog world" followed by low noise linear LDO to create -3V3 and 3V3 for opamp amplifier chain and for VDDA and VSSA of my microcontroller (STM32)

So, I got two ground plan AGND and GND, I wonder:

• It is ok to use isolated DC-DC in this case?
• Do I need to connect the two ground plan AGND and GND (in one point)?
• Can I connect the two ground plan AGND and GND nearby the VDDA pin of the STM32, it is judicious?

Answer 1

It is a good idea to use separate ground planes for AGND and GND. As you mentioned, they must be (in most cases) connected only in a single point, ideally using a ferrite bead or another kind of small inductor, so DC levels are equal but high frequency noise, coming from the digital part, is blocked from entering the analog part.

All analog parts must be near to each other, and away from digital parts, which must also be grouped together. It is good to have short paths to the supplies and use decoupling capacitors.

Isolation helps to reduce conducted noise, but it may not be necessary if noise is not too bad. Remember that isolation is from the the 24 V supply, but in any case AGND and GND must be connected at one point. Since you want negative voltages for the opamps is likely that you will have to use isolation. If you do not want isolation you could change your circuit to use single supply opamps.

For more information on how to do the a split plane grounding you can check this application note or just google something like "mixed signal grounding techniques".