This is usually done with a resistive voltage divider. For not loading down the node being measured the two resistors should be large values. This produces a problem if the Thevenin output impedance of the divider is large and makes a voltage divider effect with the ADC input impedance. You have to make a judgement between the two sources of error in your resistor value choice.
If you want to add an op amp, you can make the see-saw inverting stage with the two resistors chosen for reduced gain. This will eliminate the divider effect of the ADC input impedance problem. To get the polarity back, use a second inverting stage.