STM32 ADC readings are saturating, any thoughts?

[Rclar.13]

So we have just gone into production on a new design, a very small part of the circuit monitors the devices battery voltage fed into one of the ADC channels. I am finding that 1 in every 50 boards has developed a failure with the ADC where it just reads 0xFFF for this channel. The battery I am monitoring is divided down with a 1M potential divider, following that there is an unity gain buffer with a low pass filter on the output . The low pass filter is 470R and 100n. The battery is 4.2V to 3V and gets scaled down to 2.1-1.5V via the potential divider. my VREF+ pin is connected to an external 2.5V reference. My VDDA pins are filtered and connected to a regulated 3V and system GND. I can instrument the failed boards and all the signals external to the IC check ok ie VDDA, EXT_REf etc, input looks correct no spikes etc. The issue is solved by replacing the processor with a fresh one and the board works correctly. Any thoughts as to what is going on ?

[NVand.1]

Hi Rclar.13 and all,

I seem to be experiencing a similar issue. I am using an STM32L496ZG and during the test production run, we got 4 out of ~200 with saturating ADC measurements.

The only fix we have until now is replacing the CPU. In which case, everything is fine again.

The voltages at the input of the ADCs are measured as expected.

Rclar.13 what is the date code of the CPUs that are falling for you?

[Rclar.13]

Hello NVand.1 . This sound similar to me I was using a SMT32L4R5ZIT. The voltages at the ADC inputs are as expected but the readings are saturated. As you have mentioned replacing the cpu does fix it.

I am not sure how to find out the date code.

[NVand.1]

By the way, are you using the one with the external SMPS?