Hello,

> There are two issues here:
> 1. his input range is 0...-15V  or the range to be measured 
> is around -12V (say -10V...-15V ) ?

The nominal voltage is -12V. The voltage can have some tolarance, so
-10V to -14V is the allowed range.
On error, the voltage can somewhere between -14V and 0  

> 2. it's unknown if the source is floated or not relative to 
> the microcontroller ground

Source GND must be also PIC GND.
-12V is only one of 5 measured voltages, but all other are positive. But
all have the same reference GND. 

Best Jens

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