I think we are at cross purposes.
By converter I meant the analogue to digital converter I think you meant the voltage converter. Adding extra capacitance on the 5V rail can help but not much in my experience. If it were that easy then the manufacturers would have done it already and got a better spec. But it is certainly something you could try.
I would be inclined to go with a switch converter up to something like 9v and then use a liner regulator to bring it down. There is much less noise like that.