In the STM32F205RGT6 datasheet, there is a formula for the recommended R_AIN external input impedance for the ADC pin for lowest noise.
Page 126 of the datasheet:
The only thing that isn’t known is the k factor from the value of the ADC_SMPR1 register (page 334): http://www.st.com/content/ccc/resource/technical/document/reference_manual/group0/4f/7b/2b/bd/04/b3/49/25/DM00180369/files/DM00180369.pdf/jcr:content/translations/en.DM00180369.pdf
Can someone from Particle confirm what they set this register to in the default firmware?
I’m pretty sure the ADC_SMPR1 register is the sample time value.
The default value is 480, but you can it to other values.
Yep, those numbers line up exactly with the options in the second PDF link. Thanks!
This is the approximate input impedance of the ADC so you should drive it with a much lower impedance source to get good operation. Using a source impedance as high as R_ain will almost guarantee poor results.
This is a not a resistor you put in series with the input, as the title of the thread suggests.
Correct, it’s really a maximum, and by series resistor, it’s probably better described as the Thevenin equivalent series resistance of the source feeding the pin.