in the external ADCs (most of them), the SNR is one of the important parameters in the datasheet. Maybe for MCUs the SNR is not that much high.
What I intended to say: They give you a lot of information to do the math yourself, to calculate the SNR for your particular use case on your own. I must admit, I've never done that for my purposes anyway.
You can't calculate the SNR; you have to measure it. As pointed out by Gibson486, AN2834 provides some noise measurements for the STM32F407 - up to 24 counts peak to peak, reducing to 18 with Data+Instruction cache turned on and prefetch turned off. I got 25 counts at mid range on an STM32F407VG Discovery board. It's not particularly good so perhaps the reason it isn't specified in the datasheet?
https://www.st.com/content/ccc/resource/technical/document/application_note/a0/71/3e/e4/8f/b6/40/e6/DM00050879.pdf/files/DM00050879.pdf/jcr:content/translations/en.DM00050879.pdfSNR is often given in external ADC datasheets, and it's rumoured MCU datasheets exist that specifiy SNR (or disguised as ENOB).
Pretty much all of the more recent MCUs with >= 12bit ADCs specify the ADC noise including all STM32F3 series, L4, F7, H7, most (all?) Kinetis, SAM, MSP etc.
But - if you find such SNR spec, you must read the datasheet very carefully to understand which application case and which external circuitry this particular SNR applies to.
Difficult since I don't recall ever seeing an MCU datasheet which specifies the measurement conditions beyond temperature and supply voltage. But you can be sure that the datasheet values will be the best that can be achieved - eg. with input at or near zero so that the noise of the reference is not included.
With external ADCs, the SNR usually is given for a very ideal use case - because marketing wants to see here a better value than some other ADC.
So for some use cases, e.g. audio and RF processing, SNR is important. If the datasheet specifies a SNR for your desired use case, this gives you a good hint of how well the ADC will perform in an optimal environment. Most MCU internal ADCs aren't targeted for audio or RF processing, so no SNR is given. This doesn't imply that the internal ADC must have a bad SNR.
Noise is relevant for *all* ADC applications including DC - a 12 bit ADC with 8 bits of noise would be pretty useless for most uses. But the majority of MCU 12 bit ADCs do have the SNR specified. 10 bit or less aren't (usually), probably because the noise is typically less than 1 LSB and thus not an issue for most users. If you increase the resolution by adding dither and oversampling then the noise is relevant but you would have to evaluate it yourself.
For other use cases, you may want other specs to be good and SNR might be totally pointless, e.g. precise DC or LF signal measurement, or when you must get an accurate as possible single sample of your input, or you need a very fast settling to high accuracy. SNR spec won't help you here, you'd look at stuff like long term reference / gain and offset stability that aren't covered by SNR at all.
SNR is almost never pointless - even in a DC measurement, resolution is limted by noise. You can oversample and filter (eg. by averaging) to reduce noise and increase resolution but this is limited ultimately by
a) flicker noise which increases at low frequencies at the same rate that increased averaging reduces it and
b) correlated noise that can't be reduced by oversampling.
For example I configured an STM32F407 ADC on a Discovery board to 2.4MSPS with the input driven by a two 800 ohm resistors as a voltage divider from Vref. Averaging by 10^6 reduced the peak to peak noise from 25 LSBs to .01 LSBs, .00163 rms or 18.6 noise free bits. Averaging by a 32 x 10^6 reduced the pp noise .0041 LSBs, .001 rms or 19.94 noise free bits.
The latter would be good enough for a 5 1/2 digit meter but INL and DNL limits accuracy to around 10 bits, but you could measure the INL and compensate for it to drastically improve accuracy. INL will vary with temperature so it wouldn't be easy to maintain accuracy over temperature and time.
The DNL/INL specs will give you an idea of how precise the ADC can work, this has significant influence on SNR but doesn't cover noise.
Errm, SNR is 'signal to noise ratio'. How does DNL/INL influence, which are static, DC specifications, influence SNR?