filter on character fields in ALV grid report

Hi all,

I have done a report which has vendor number, cost center and wbs element using ALV grid display. I have declared all the fields as in the database fields. when filter is applied on these fields the data is not retrieved. Only after padding zeroes to the input data i get the data.Is there any way to avoid entering the zeroes manually? I know we can use the conversion exit alpha function module , but how will that apply to the filter? If any one is aware please help.