LIS3DH ADC1 input troubleshooting

[CALAY]

I've implemented a new low power design using LIS3DH, and I had in mind to use ADC1 input to monitor the battery voltage and ADC3 to monitor temperature.

I succeed to properly read all three accelerometers axis as well as temperature. On the ADC1 input if I connect directly a voltage controlled source I've got consistent measurment form about 0.8V up to 1.6V, but when I connect the resistor voltage divider (27MOhm pull up and 100MOhm pull down) with a divided voltage around 1.1V the measure clamp to -32768.

I did'nt succeed to find any information about ADC1 inpu impedance, behaviour our any application note.

The high impedance divider is setup to reduce standby current.

Any help would be gratefully appreciated.

[Eleon BORLINI]

Hi @CALAY​ , the ADC input is a high-impedance node (should be, anyway...). I'm wondering about the dynamic of the ADC1... Which is your LIS3DH Vdd? Because if Vdd=1.8V the dynamic of the ADC is about V = 0,6 × VDD ± 0,29 × Vbng, and maybe the 1.1V of your divider could fall down this limit... You could try to increase the partition voltage or use the ADC2, which dynamic is V = 0,88 × Vbng ± 0,29 × Vbng. Regards

[CALAY]

Thanks for your reply Eleon

Vdd is 3.3V

According to resistor devider the voltage on ADC1 input with 4.2V battery should be 3.3V but it is only around 1.1V, that make me think that ADC1 input impedance is about 10MOhm.

Nevertheless, with this 1.1V ADC input generated by a controlled (low impedance output) I get consistent measurement, and the same voltage with high impedance resistor divider bring me back bad results.

Any idea?

[CALAY]

Could you please point me out the documentation where you found this V = 0,6 × VDD ± 0,29 × Vbng formula

It could be helpful