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Analog and Digital Ground Separation

Felix889 , 02-10-2021, 05:22 AM
Hi Robert,

I am currently working on a very dense design, which includes A/D converters, PIC32 microcontroller, a lot of FETs to enable all the ICs we are using, CAN Interface, Op-Amps, High Current protective switches (Total Current is about 125A). The board will be used for Automotive application and there will be a of switching happening at the same time to enable various sensors, contactors, FANS, Pumps, Motors, Valves, Analog data from some sensor will be sent to A/D converters and then back to the microcontroller, as well as FAN speed… We have both Analog and Digital signals, they are not high speed, and from what I understood watching your and Ricks Webinars, that there is no need to separate Analog and Digital Grounds. I just need to leave the gap between them big enough (20H), so they would not interfere with each other. That means that I cannot use any datasheet layout guidelines because they strongly recommend splitting the Ground Plane. Rick suggested to use like 1k resistor between FETs from my Analog circuitry and a controller on the Digital side and place it on the same level as my A/D converters, but I am not sure if I need a 1k resistor on the input of a MOSFET because that will limit the input current from the microcontroller. My biggest concern is the input power, which is driving those ICs. It is common for both Analog and Digital circuitry (+3V3 and +5V) and I am not sure what is the best way of separating them. That is the most interesting question, what is the best way of separating common Analog and Digital ICs power, as it might not necessarily be routed using power planes, it can be routed using wide tracks if the space is limited and what to do in that case?
Is it a good idea to route a GND trace between them so their fields would couple each other, but if they are coming from the same source that probably would not matter because at some point they will be back to the same source unless we separate them using a capacitor.
If I have some PWM signals coming from my controller to Op-amps to change a voltage level and these signals go to a connector. I am not sure if I need to put that circuitry alongside A/C converters (to keep all Analog circuitry together) which will have sensitive Analog data or the best practice is to separate them as well.

I would appreciate it if you could advise me on that.
Thank you.

karimoviç , 02-11-2021, 01:01 PM
Hello , may be you could separate the AGND and DGND and make a resistance or capacitor with a low value between the 2 GND plane , and when you receive your board you can test the two cases : AGND and DGND coupled with the low value capacitor/resistor and the case where they are separated (without the shunt resistor/capacitor) and see if there is a big differnce between the two cases .
robertferanec , 02-15-2021, 12:49 AM
@Felix889 it looks to me, that you are aware of possible issues. You may just need to try to design it the best way as you think it should be designed + if there are some things you are not sure about, you may add some options for additional components or connections - that is what I normally do.

Maybe someone from forum can help you answering something more specific?
Felix889 , 02-15-2021, 01:26 AM
Thank you Robert for getting back to me. Do you normally use the same power supply (for instance +3V3) for both Analog and Digital circuitry with some kind of separation like a resistor in the image provided below or maybe you use one PSU for each circuitry? The interesting question then is where is the best place on the layout for those resistors, shall we place them on the same level as A/D converters or somewhere else? Thank you.

robertferanec , 02-15-2021, 01:46 AM
For some reason I can't open the PNG files.

I do like to use separate DC/DC power supplies for analogue and for digital powers. It depends on currents, but I like to use linear DC/DC for analogue - for some reason I imagine this as a more clear power than output from a switching power supply. But I have never done any measurements to confirm that.
Felix889 , 02-15-2021, 01:51 AM
I have again uploaded the PNG files for you to see. Thank you.
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