Hi,
I need to measure a 4.2V battery while my MCU's supply varies between 2.0V and 3.3V. Since VDDA is the ADC reference, raw ADC counts will drift as the supply changes.
And I don't want to use any external references.
My plan:
- Feed the battery voltage through a 10K/7K5 divider into one ADC channel (scaled to stay below VDDA_min = 2.0V).
- Read VREFINT on a second ADC channel.
- Use VREFINT (whose true value is known from the factory calibration in system memory) to back-calculate the actual VDDA, then convert the battery channel's raw count into volts.
The formula being:
V_battery = 3.0 V × (VREFINT_CAL / VREFINT_DATA) × (ADC_battery / 4095) × ((R1 + R2) / R2)
Is this the correct approach for getting a supply-independent battery reading on this part?
Anything I'm missing... sampling time for VREFINT, divider impedance vs. ADC sample-and-hold, or calibration register quirks specific to the C0 series?
Thank you
