Popup always stays on top

I've WPF application which has one Main window on which I'm opening a Popup window. The problem I'm facing is the Popup window always stays on top. If I open some other application(any windows application) the main window goes into background but the Popup windows remain on top.However if I minimize the Mainwindow the Popup also minimizes.

Please help on overcoming this issue.

Update:

I am opening the Popup as below

 myPopup.IsOpen = true;