AGND plane connection to DGND - Page 1
Forum section: electronics / beginners

author: Spitfire1.2
author_rank: Newbie
timestamp: February 21, 2020, 06:55:48 am
content: Hello!I am doing a project where I have to connect 9 2.7VDC analog thermometers on an ATXMEGA128A1U among other things. As far as I knew, when doing an analog ground plane you should connect it to the mcu's agnd pin. The mcu in this circuit doesn't have a pin designated as agnd. So I used a narrow corridor to connect the two planes close to the mcu.I am not sure if this is completely right or wrong, so I would like your opinion if possible.Thank you for your time!

author: Neilm
author_rank: Super Contributor
timestamp: February 21, 2020, 05:06:35 pm
content: Personally, I never split the 0V plane unless there is a really good reason - i.e. safety isolation.I have seen a lot issues with split planes causeing problems if the designer has not paid very close attention to where the current loops are going.

author: thm_w
author_rank: Super Contributor
timestamp: February 22, 2020, 01:27:00 am
content: As noted by Neil, usually its better without. But, the way you've done it is correct and seems low risk. Although I would enlarge the fill in the bottom left so it goes underneath the traces on the top layer.Some other ideas:- remove the thermal reliefs on vias- consider using silkscreen to designate capacitors/resistors body, so its obvious where they should be placed- add more ground underneath the trace at the top, maybe going from top left to top right then downThere is probably more in terms of the actual atxmega schematic that will have an effect, eg what reference is used, avcc decoupling, filtering, etc.

author: ve7xen
author_rank: Super Contributor
timestamp: February 22, 2020, 01:57:11 am
content: As has been noted, if you don't know why you're splitting the ground, you probably shouldn't. Where you place the ground plane connection depends a lot on where the currents are flowing in your circuit and which points 'matter'. Splitting the ground is kind of like creating a star ground point where they join, such that every ground pin sees as close to the voltage at that point as possible, with minimal influence from other currents. The primary reason you do this with digital and analog grounds is so that relatively high digital currents with fast edges aren't influencing the ground voltage seen by the ADC or sensors, and in this aspect, it doesn't matter much where you join the planes as long as there's only one place where they join. There are two additional considerations (well, probably more) - first is that you don't have a dedicated analog ground pin in this case, so you can expect some digital currents to flow into its ground plane. Since this is unavoidable, you probably want to aim to minimize the distance between this pin in particular and the 'star ground' point where the planes join, to keep the ADC ground and 'reference' ground of the star point as close as possible and minimize the voltage change on the analog plane created by these currents. The second is that this point becomes the 'reference ground' for your system, so in particular you want your ADC's ground reference to be as close to this voltage as possible, so placing it near the ADC makes sense. If you were to place it elsewhere, the power supply currents flowing will affect both the ADC and sensor ground reference voltages instead of just the sensors, and likely in a less symmetric way, too.Analog have some pretty good posts on the topic that I suggest you read:https://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.htmlhttps://www.analog.com/en/analog-dialogue/articles/grounding-again.htmland probably some others.None of this is likely to matter using the ATmega's internal 10-bit ADCs, though, and simply splitting the plane to avoid digital currents causing ground bounce and the like is going to have the biggest impact, but even then with this level of precision it probably won't matter unless you've got some fairly serious currents flowing. Considerations of power supply and AREF are probably more relevant.I would place the ground plane join directly at the GND pin closest to the microcontroller inputs that you are using for the ADC. I don't see the micro in either of these screenshots so not sure where that is, but it doesn't look like where you've put it. Widen/extend the plane so it covers all the analog traces all the way to their microcontroller input pins.

author: Spitfire1.2
author_rank: Newbie
timestamp: February 22, 2020, 01:09:25 pm
content: This where the micro is, I've circled the mcu/ground connection via closest to the joint (i hope this makes sense). This is my first pcb at this scale, so please be gentle

author: Spitfire1.2
author_rank: Newbie
timestamp: February 23, 2020, 12:18:38 am
content: The circuit also has 2 SPI devices and a parallel SRAM, so that's why I thought splitting the planes is necessary.