STM32WB55 ADC Internal Resistance Issue

[kickerkro]

Hello STM Community,

I’m currently working on measuring a LiPo battery voltage using one of the ADC inputs (AIN6) on an STM32WB55RG. The battery voltage ranges from approximately 3.0 V to 3.95 V. Since the ADC reference (Vref) is 3.3 V, I used a voltage divider (two 10 MΩ resistors) to scale the voltage to half its original value. In firmware, I then multiply the measured voltage by two to recover the actual battery voltage.

However, I’ve observed that the measured ADC voltage is roughly three times lower than expected. This suggests there may be an additional internal resistance or loading effect influencing the measurement — possibly an internal parallel resistance within the ADC input path. I checked the datasheet and reference manual but couldn’t find any specifications confirming the presence or value of such an internal resistor.

I’ve attached my tentative schematic for reference.
Has anyone encountered a similar issue or can shed light on this behavior? I’d like to make the measurement as accurate and robust as possible, and I’d appreciate any insights into why this might be happening.

Best regards,

[TDK · Accepted solution]

5 MΩ impedance is incredibly high. If you slow down the ADC clock and increase sampling time to max, it can work.

See section 3.2.7 Effect of the analog signal source resistance here:

How to optimize the ADC accuracy in the STM32 MCUs - Application note

[kickerkro]

The reason to choose such high resistance is to prevent static current flow through it and save power. Changing the ADC clock and increase the sampling time does not help unforutnately. I think I will try to manipulate the data in the firmware although it doesnt sound very reliable.

[MasterT]