I'll let you work out the details for your particular case, but I'd like to add some more clarity.
First off, I've opened up an errata report on RM0410 (the reference manual for my chip) here.
I'll be referencing Reference Manual RM0410 Rev 4, which is the Reference Manual I need for my case. This isn't for your chip. You'll need to find the one for your chip.
ST's chips allow you to choose an ADC sampling time AND resolution.
RM0410 p473. Here are some sampling time options for me. They range from 3 to 480 ADCCLK cycles. I've highlighted them below:
They are:
3 cycles
15 cycles
28 cycles
56 cycles
84 cycles
112 cycles
144 cycles
480 cycles
You can see these from the drop-down menu in STM32CubeMX:
Each clock cycle is equal to one ADC clock cycle, called ADCCLK in the RM. The equation for ADCCLK frequency is:
ADCCLK = PCLK2 / prescaler
Where prescaler can be 2, 4, 6, or 8.
Use STM32CubeMX to see these prescaler values from the dropdown menu as well. You'll notice that PCLK2 divided by 2 is greyed out for me, meaning it is an invalid value for my configuration, probably because that would result in an ADC clock that is too fast.
Conversion time is based on the bit-resolution you've set for your ADC. Options are 12 bits, 10 bits, 8 bits, or 6 bits.
Conversion time based on ADC resolution is as follows:
12 bits --> 12 ADCCLK cycles
10 bits --> 10 ADCCLK cycles
8 bits --> 8 ADCCLK cycles
6 bits --> 6 ADCCLK cycles
The RM is a bit misleading (hence my errata report above) but here's what RM shows on p452. Take a look at my notes for clarity and corrections:
Putting it all together, the equation is:
total_ADC_sampling_time = user_chosen_sampling_time + conversion_time_based_on_resolution
where
conversion_time_based_on_resolution = 1 ADC_clock_cycle/bit_of_resolution x bits_of_resolution
And again, as shown above,
ADCCLK = PCLK2 / prescaler
Examples:
Example 1: Let's say I'm doing 12-bit resolution and 480 clock cycles sampling time
My total sampling time is:
total_sampling_time = 480 + 12-bits x 1/bit = 480 + 12 = 492 ADCCLK cycles.
Assuming PCLK2 is 108 MHz (see "Clock Configuration" tab in STM32CubeMX), and prescaler is 8, we know:
ADCCLK = 108/8 = 21 MHz.
1/21 Mhz = ~47.619ns/clock cycle
Total sampling time is therefore:
492 ADCCLK cycles x 47.619ns/clock cycle = 23.429us
Sample rate is
sample_rate = 1/23.429us = 42.682 kHz.
Example 2: What's my absolute maximum sample rate possible at 10-bit resolution?
For my case, STM32CubeMX greys out the PCLK2 divided by 2 option, so the fastest I can choose is a prescaler of 4. The fastest user-selectable sampling time possible is 3 cycles, and 10-bit resolution adds 10 more cycles, for a
total sample time = 10 + 3 = 13 cycles.
ADCCLK = 108/4 = 42 MHz.
1/42 Mhz = ~23.8095ns/clock cycle
Total sampling time is therefore:
13 ADCCLK cycles x 23.8095ns/clock cycle = 309.5235 ns
Sample rate is
sample_rate = 1/309.5235 ns = 3.230772 MHz! <=== very fast for a mcu!
Which ADC sampling time should I choose?
I mean, why all the options?
Here are the available sampling times to choose from:
3 cycles
15 cycles
28 cycles
56 cycles
84 cycles
112 cycles
144 cycles
480 cycles
Answer: there isn’t really any info on this in the ST Reference Manual that I can find, but essentially, sampling an analog signal more slowly reduces surge "parasitic" drain on the analog line as a result of the sampling process itself, which both draws a tiny current from the analog line and takes time. If your analog line has an extremely small current source capability (think: the analog source "recharging" the capacitance of the analog line can only produce a tiny tiny current to recharge that analog line), then it cannot handle a high sample rate sampling on it. This is, again, because the sampling process itself will draw current, making the analog signal sag (distorting it from its true value), and introducing noise to the analog signal. In such cases you are wise to choose a slow sample rate by setting your sample time to a very long value, such as 480 ADC clock cycles, for example, which is the longest you can choose.
Rule of thumb:
In short, just choose the longest sampling time you can, to be safe, that will meet your sample rate requirements. If you must sample as fast as possible, do so, increasing sample time (and therefore decreasing sample rate), as required, until your analog signal remains stable and stops fluctuating due to noise you are adding to the analog line as a result of your sampling.
Make sense? Go slow unless you need to go fast. If you need to go fast, ensure you're not going too fast. If you're going too fast, slow down until you're no longer going too fast.
Done.
