Thank you for your help.
I discovered that the problem is not with EMI but with power supply: Using double power supply (e.g. Wixel powered by usb, or powered by a separate VIN source - obviously with common ground) the noise completely disappears. (The noise given by the opamp is about 30 mVpp)
Maybe the capacitors could not be sufficient, so I changed them: instead of C2 = 220uF I put a big 2200uF capacitor. In addition of it, I added another 2200uF soldered on VIN and GND pins of wixel and a 470uF capacitor soldered on pins 4 and 7 of the OPAMP. In this way the disturbance decreases: from 2 Vpp of the initial condition to 150 mVpp.
The circuit works with Vcc = 6V, 3V3 is used only for biasing the signal. I know limitation of the opamp, but when I was testing the preamplifier I never encountered these kinds of problems. Do you think that the two phenomena (low voltage supply and disturbances given from the istantaneous current request of wixel) could be related?
I can try to replace ua741 with icl7612, but I don't know the role of Iqset pin on icl7612...