STM32wb analog voltage referrence and VSSA

[HSpre.1]

Hi there

I am currently developing a product and i would like to use either stmb32wb55ce or stm32wb55rg

I have done most of the softwaredevlopment on the nucleo board with stm32wb55rg.

I am a bit stuck with the analog world on this MCU.

I would like to use a 2.5V reference and use ADC.

I would like to use the internal 2.5V reference since if I go into standby using RTC to save power and only wake up every 15 minutes or so.

I am uncertain as to how VSSA and VREF+ and the VREFBUF should be connected even after going through datasheets.

Queations

1) I wanted to use STM32wb55ce but according to datasheet VREFBUF is unavailable thus my understanding is i cant use internal 2.5V reference for my adc?

2) If i rather use stm32wb55rg I can use vrefbuf and set to 2.5V. IF i do this do i connect Vref+ to VDDA? and ad external 1uF and 100nf cap? Or are they internally connected and i just add 100nF and 1uF to each pin?

3) for stm32wb55RG there is no VSSA pin , how is VSSA connected to GND pins? I normally use seperate analog gnd and connect at power input GND pin with 0ohm resistor to make sure there is as little gnd noise on VSSA as possible.

Thanks in advance for help

[HSpre.1]

I think I understand now , please help me if I am incorrect.

For the 48 pin package:

We cannot us VREFBUF. since VREF+ is connected to VDDA. You have to use external reference which I prefer not to with battery environment and power saving.

VSSA is not brought forward on the pins.

Question: Is VSSA internally connected? 0 ohms?

For 68 pin package:

For me to use vrefbuf 2.5V reference.

I will connect 3.3V to Vdda with LC filter.

I will enable VREFBUF and set divider to 2.5V and add 1uF and 100nF cap.

VSSA is not brough forward on the pins.

Question: Is VSSA internally connected? 0 ohms?

VSSA internally connected to VDDS is a bit problematic for creating separate quite ground plane with proper star ground connection point.

[Remi QUINTIN · Accepted solution]

On device package which do not have a dedicated VSSA pin, VSSA is internally connected to VSS.

[HSpre.1]

Thank you. It does make sense even though I cannot find( or overlooked) direct confirmation of this in datasheet.

I'll have to simply do pcb layout for my star ground to be "within" stm32wb55rg. Since connecting my star ground plane at 0V input of board and at cpu will create issue.

[Remi QUINTIN]

​You can see the confirmation in table 70 "ADC input/output pins" of the RM0434 , chap 16.3.2.