Level shifting in OPAMP and HE100T01 interfacing problem
On most op-amps you cannot properly use an input signal that is below the - supply pin. You might try to bias the sensor output to 2.5v (with equal pull up and pull down resistors on the sensor's output).   

Alternately, to set up a true level shifting circuit see these examples that use hall effect sensors:  

http://www.next.gr/sens-detectors/hall-effect/current-monitor-l13368.html

http://www.instructables.com/id/How-to-Measure-AC-Current-using-Hall-Effect-Sensor/