Excel 2000

In A workbook I declared a an object variable of type ADODB.Connection in
the declarations part of a module as follows:

Public pspDB as ADODB.Connection

All functions and procedures using this variable work fine as long as the
workbook is saved as .xls.

However if I save it as Add-In (.xla) the variable seems not to be
recognized as such. It's value seems to be an empty string (""). Before
Initializing the variable I expect 'Nothing' as value.

Can anybody give me a hint what I missunderstand here? Many, many thanks,
Etienne