Solder Bridge SB60 links AGND to the Board Ground (0VDC) (ref user Manual schematic)

Addition to Martin's post: Look at PDF inside the schematic archive www.st.com/st-web-ui/static/active/en/resource/technical/layouts_and_diagrams/schematic_pack/nucleo_64pins_sch.zip

By the way, separate ground planes is a hardware thing, not software. It is used in PCB design to eliminate (as much as possible) noise propagation through different parts of circuit, especially to protect analog input circuits where weak signal of interest is.

Cheers Miloje

ok.. thank you guys!

so you mean that i can just unsolder the fuse on SB60 and thats it?

You need to connect it somehow. Either to another power supply source or to the original source through a noise filter (a LC filter for example)

i have another question, what is the function of the fuse ? i took it off and there is still continuity between AGND and GND but now the adc does not measure well.. any help please!!

In the old days before A/D converters were integrated into the MCU you would keep the analog ground and digital ground separated from each other and then tie them together at the A/D converter. You will still need to tie the grounds together. However, as one commenter already stated you could experiment with an LC filter between the grounds in an attempt to attenuate digital noise on the analog ground. Managing ground currents in a mixed analog/digital system is both an art and a science. I would recommend looking over some application notes on the topic from the analog masters: TI (try to find the Burr Brown app notes), Analog Devices, Linear Technologies, Maxim.