I'll make a few assumptions:
1. What you have is a DC motor generator.
2. The generator's signal is similar to fullwave rectified positive voltage in the forward direction
3. The generator's signal is similar to fullwave rectified negative voltage in the reverse direction
4. The signal range forward is 0 to +500 mV.
5. The max signal reverse is 0 to -500 mV.
Perhaps a circuit like this will allow voltage measurement in either direction of the windmill. Here, the voltage should be at 2.5V with no wind, 3.0V at max. forward direction, 2.0V at max. reverse direction of the DC motor. This circuit would retain direction information. 10-bit ADC should vary by ±100 (512 for no wind, 612 max fwd, 412 max rev).