I'm using a resistive divider to measure a voltage from a LiPo battery. When I connect a voltage divider with ratio 4:1 to P0_0, I read only about 0x20 from the ADC regardless of what voltage I put into the resistor divider. This is much lower than the expected value.
If I probe the input to the ADC with a multimeter, the ADC starts working and measuring values that match what I expect. When I stop probing with a multimeter, the ADC stops working again. I have tried dropping the total impedance of the divider from 10Mohms to 200kOhms, and I get the same behavior.
If I feed a specific voltage to the ADC pin from a DC power supply, the ADC seems to work correctly.
This behavior seems similar to the problems that hrg has been having here: http://support.dialog-semiconductor.com/adc-interfacing-problem
Is there anything special that I need to do to use the ADC with a standard resistor based voltage divider?
Have you done this succesfully?
Is there a maximum load impedance that should be used for the divider?