Mixed signal layout

[sert]

Hi,

do you have some recommendations for a mixed signal layout? I have already read some of the threads regarding analog and digital grounding. Also about your applications where you use LDO for analog supply and seperate analog and digital components and signals from each other on the pcb.


In my case i get an analog signal from a sensor. This is measured by a µC via ADC which then controls a gain stage to amplify the signal depending on its amplitude. So i have an analog signal entering the board, being measured by a µC and being amplified alongside with other digital components on the board.

I attached a sketch of the set up. mixed signal set up.pdf


The signal runs with a frequency of 7.5 MHz with an amplitude of about 100 to 800mV.

Would you recommend a 4 or 6 layer board to separate the analog and digital signals and components from each other? What else should be considered for such an application?

Thanks in advance.


Best Regards
​

[robertferanec]

Hi sert, I do not have much experience with precise analog circuits. However I would probably try to find some recommended designs, reference layout and white pages about AD converters for this kind of signals and frequency. They may have some recommendations.

For example: http://www.analog.com/media/en/techn...es/AN-1142.pdf

"
The most popular question among analog signal chain designers is: should the ground plane be split into a AGND and DGND ground plane when using an ADC? The short answer is: it depends.
The long answer is: not usually. Why not? In most situations, a split ground plane can cause more harm than good because blindly splitting the ground plane only serves to increase the inductance for the return current.
...
Good circuit partitioning is key to not splitting ground planes as shown in Figure 9. Notice that if a layout allows you to keep the respective circuits in their own areas then there is no need
to split the ground. Partitioning this way allows for a star ground, which keeps return currents localized to that particular circuit section.
"

PS: I am not sure what exactly you do with the signal, but maybe I would consider to use a proper AD converter instead of connecting AD of a uC to 7.5MHz signal. However it depends on application and uC.

[sert]

Thanks for sharing this information.
I went on with partitioning digital from analog and used one common GND. I also included LDOs which sit in the analog area to further improve noise reduction in supply management.