Use an op amp to remove negative signal
How would I configure an op-amp to filter out the negative portion of
  a signal.


Just use a precision half-wave rectifier circuit: -



This one also inverts so to keep the positive half of the signal whilst discarding the negative half, preceed the recision rectifier with a normal inverting op-amp stage.

You will probably need to choose an op-amp with a gain-bandwidth product that is 10 times that of your signal to get reasonable fidelity. Below is a slightly more informative circuit showing both input pins: -



And yes, as Scott has pointed out in a comment a small capacitor across D2 usually helps (about 1 pF to 10 pF is a good starting value.