ADC for measuring 0-5V but connected to 3.3V device
This can be done with three resistors!







simulate this circuit &ndash; Schematic created using CircuitLab

No active components needed. Shifts -4 to 4V to 0-1.8V, perfectly linearly. V2 is your input voltage (-4 to 4V). V1 is just your 1.8V rail.

The input impedance is 22K, which won't load your motors to any significant degree, and the output impedance is about 5K, more than low enough to drive any ADC.