All about the Ain inputs on the accessory connector.
Each input connects to a Pi IO Pin and an ADC input. Clearly you can’t use both. There is also an ESD protection diode and 33ohm series resistor to reduce the chance of damaging the input.
To use the ADC set the corresponding GPIO to input with pulldown.
The max input is 3.3V so use a resistor divider above that voltage. The PS Input is scaled with 48.7k and 10.0k 1% this gives a range of:
The actual installed ADC is a TI I2C 4CH 12Bit Device It is SW compatible with the ads1015 with a few less features.
The resistor values are set into SW so the sensor command displays the voltage correctly.
pi@compass:~ $ sensors
Adapter: bcm2835 I2C adapter
User ADC Differential: +0.00 V
+12V: +13.77 V
User ADC 1: +0.00 V
User ADC 2: +0.00 V
If you put 10.0k and 10.0k in your signal path, you can read sensors and multiply User ADC by 2.
It is possible, but not trivial, to change the file so it reads directly with your values. This will be documented in a future APP Note.