Play-hookey.com (a very useful site) has the following to say about your circuit:
"The circuit (attached below) shows a non-inverting op amp circuit. In this circuit, the input signal is effectively used as the reference voltage at the "+" input to the differential amplifier, while the "-" input is indirectly referenced to ground. In order to keep the two input voltages to the amplifier the same, the amplifier must set Vout to whatever voltage is required to make the feedback voltage to the "-" input match the input voltage to the "+" input.
Since Rf and Rin form a voltage divider, the feedback voltage will be VoutRin/(Rf + Rin). The gain of this circuit, then, calculated as Vout/Vin, is (Rf + Rin)/Rin, or (Rf/Rin) + 1. Resistor Rz has no effect on the gain of the circuit. However, to balance out variations caused by the small input current to the amplifier, Rz should be made equal to the parallel combination of Rf and Rin."
Referencing your "-" input to ground may solve your problem.