Thanks for your help.
Ok, the leakage of the cap is higher than the input bias current of the op amp, so what is the effect ? since my RC filter is placed after a voltage reference (that I would want to filter), what can be the problem with the leakage current of the cap and the input current of the op amp ? this may produces an offset, but I don't care about it... is there any other effects that I could forget ? if there is a current which is sink by the op amp and the cap (leakage) the voltage reference could provide it ... isn't it ?