What's the input impedance for the ADC on both the EFM32 and (EFR32 series 2)? In the datasheet for the EFM32HG, section 4.10 for the ADC, it lists I_ADCIN < 100nA, suggesting an impedance of >10 MOhm, but then it also lists R_ADCIN > 1 MOhm. In the EFR32FG22 datasheet, I didn't see a impedance spec in the ADC section.
The use case Is I want to monitor battery voltage in a 2s lithium cell. So I'd measure the voltage every few seconds and use a voltage divider to bring it down to the ADC range. I want the voltage divider resistors to be as large as possible to minimize current loss. I think 8-bit ADC resolution is probably fine
You could take a look at this KBA. It was written for EFM32 S0. But the structure may also apply to EFM32/EFR32 S1 and S2.
IIRC, the impedance is at the level of 2.0 k ohm for S1. I would think you could estimate similar level of impedance for the S2 if you don't need very accurate impedance.
The I_ADCIN is more like I/O leakage level.
The data R_ADCIN > 1 MOhm is wrong. you could take a look at PRCN of EFM32LG.
Unfortunately I did not see this kind of PRCN for EFM32HG. But basically I think this is same document issue.
You could take a look at Application here, the theory background may also apply to EFR32.
The high external resistor (voltage divider) may need you to use high acquisition time. the leakage may cause some inaccuracy.
If possible, you may consider to add an external analog switch (on the voltage divider). when you need ADC capture, you can turn on the switch, when not, you can turn off the switch, this may need a dedicate GPIO. You may need to take the resistance of the switch into consideration when you calculate the voltage.