Converting negative DC voltage to positive equivalent
If your input ranges from say 0 volts down to -32 volts then you can use a simple inverting op-amp configuration: -



Consider the op-amp chosen will work well with -in and +in close to 0 volts. This is not unusual with rail-to-rail op-amps. Next, as a simple mind-experiment imagine that R1 is 32 x R2. So if Vin is -32 volts then Vout has to be 1 volt to drive -in to 0 volts. I use the word "drive" because this is what happens in an op-amp with negative feedback - the output drives the -in input to match the voltage at +in. 

No negative voltages are required to be produced by this op-amp configuration. So, if Vcc is (say) 20 volts, you could make R1 twice as big as R2 and this would convert -32 volts at Vin to +16 volts at Vout. -16 volts at Vin would produce +8 volts at Vout.

There are a few op-amps that work very close to 0 volts but, as always, to widen your scope of choice, it might be better to make a small negative rail (-3 volts is all that is needed) to power the op-amp's negative supply pin: -