BlueNRG-1 ADC input with higher voltage than VBAT sinks a lot of current

[rastik]

According to the datasheet, it is possible with ADC to sample voltage up to 3.6V, even with lower VBAT, if proper attenuator.

I'm powering the MCU with 2.5V and feeding battery (up to 3.6V) to ADC1 pin. No matter what setting I use, the ADC is still sinking a lot of current - approx. 40 uA for ADC1 pin and more than 1 mA for ADC2 pin.

I've even tried setting CHSEL to 0 that should result in input connected to the ADC, but it is still the same.

My standard setup is ADC1, CHSEL=2, OSR=0, single mode, REFSEL=2, PGASEL=2.

Is there anything I'm missing in the specs?

[Ozone]

> Our problem occurs when VBAT < VADC1 < 3.6V.

Again, from experience with other STM32 MCUs, in my case here the F303x (DocID023353 Rev 13).

It states for instance:

3.7.1. Power supply schemes

The VDDA voltage level must be always greater or equal to the VDD voltage level and must be provided first.

And under 6.3.1 General operating conditions

V(IN) : Min = -0.3V, Max = VDD+0.3V

Thus, the applicable voltage limit depends on VDD/VDDA.

I vividly remember a total "meltdown" of PCF8591 chips (a 5V ADC with I2C interface) in one of my former company's applications when the ADC input voltage exceeded 5V.