PCB grounding: analog, digital & theirs supply

[mike_piw]

Hi everyone,

I'm designing PCB witch is kind of extension for STM32 Nucleo H743ZI.
One of the most important funcionality for this set is ultra-low noise measurement of few 0-10V analog signals (each input with op-amp buff).
There will be aslo some digital pheripherials (eg. spi & uart devices) on PCB.
Whole set will be supplied from external PSU +18V, so I need 2 stabilizers: one 5V for Nucleo and digital stuff and 12V for analog buffs.

My question is about correct grounding for ultra-low noise analog mesurements. I'm familiar with ideas of star-grounding and seperate analog & digital ground.
Fortunetely Nucleo have AGND pin for analog measurements, so GND from op-amp buffs should go there, but I'm not shure how to connect GND of supply for those buffs.
This supply inclueds LC filter, 12V stabilizer ans some cap's.

Which point of ground is correct for this purpose: AGND Nucleo pin (with op-amp buffs) or input PSU plug with all other (for digital lines aslo) stabilizers? Or maybe other way?

Thanks for any advice!

 

[KlausST]

Hi,

one can not calculate with "ultra low", you need to give values.

I recommend to use differential wiring of the "low noise" signal

Where and how to ground depends on the application, especially currents, loops, filters, signal sources, noise sources ... thus it can´t be answered generally.

Fortunetely Nucleo have AGND pin for analog measurements, so GND from op-amp buffs should go there,

not necessarily. I guess oyu talk about GND of OPAMP supply, but you need to take more care about GND as analog reference in signle ended signals. The GND current often (but not always) should flow any where.

I recommend to show us the complete signal flow, the grounding system, the supply wiring ... and so on.
A textual description will be too vague.

Are you more concerned about DC performance or AC performance?

Klaus

 

[danadakk]

You might take a look at this. Its not your part but principles the same. This
board results in 20 bit performance.

https://www.cypress.com/file/198511/download

Also not all caps equal in bypass performance for same value cap -

Note OS-CON is polymer tantalum.

https://www.analog.com/media/en/technical-documentation/application-notes/294542582256114777959693992461771205AN280.pdf

https://www.ti.com/lit/ml/slyp167/slyp167.pdf

Regards, Dana.

 

[stenzer]

Hi,

have a look at this two documents [1], [2], which hihghlight the return current path across a GND plane. Have a look on the DC and AC case. Depending on the literature, a frequency of > 10 kHz is mentioned, where the return current takes the path of the lowest resistance. So keep care wher your current migth flow.

[1] https://www.analog.com/media/en/ana...6/number-2/articles/staying_well_grounded.pdf
[2] https://www.analog.com/media/en/ana...cing-ground-bounce-in-dc-to-dc-converters.pdf

BR