3.3V regulator overhead and ADC reference choice

[ajs123]

Hi,

I'm using the 3.3V from a Feather nRF52840 Express as power for a potentiometer position sensor, for a device that's most often powered by a LiPo battery. Right now, I'm using the fixed 3.6V ADC reference. As would be expected, readings are off when the LiPo battery is almost dead, as the regulated 3.3V begins to sag.

Question: What's the overhead for the diode and 3.3V regulator on this board? The adc_vbat example provides voltage vs. remaining charge (e.g., 3.60V = 10%). Knowing the overhead will help to determine whether to worry about this at all.

Advice: I need a low battery indicator :-). I'm considering also switching to the Vdd reference for the ADC, so that the potentiometer reading tracks the sagging 3.3V. I can then either adapting the Vbat reading which will now be relative to Vdd near the end (maybe complicated) or switch between references as needed. The example indicates that a delay of only 1 ms is needed for the ADC to settle after a reference change, but others' thoughts/experience with that are appreciated!

If of interest - the project is github.com/ajs123/KBikeBLE. The pot is measuring the magnetic brake position on a spin bike. Near battery end of charge, the brake position measurement is "lighter" than actual so the symptom is "why is this so hard?".