ADC sample rate for best series resistor practice on Electron

Firmware
1

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?

2

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.

https://docs.particle.io/reference/firmware/electron/#setadcsampletime-

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3

Yep, those numbers line up exactly with the options in the second PDF link. Thanks!

4

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.

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5

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.