Sharepoint custom page Full postback with SPGridVIew depite the presence of update panel

Question

Hi all

I have a custom sharepoint aspx page that has a dropdownlist and a SPGridView.

I put the dropdownlist inside an update panel so when the selected index changes the page does not refresh.

It worked fine till I added a SPGridVew to the page with an ObjectDataSource as a datasource for it. after I added the SPGridView the page made full postbacks when the dropdownlist selection changes despite it is inside an update panel

what is the reasin for this nand is there any workaround for this ?

thanks