For a DDIC table, you should use DDIF_FIELDINFO_GET which is a released FM, better than DB_GET_TABLE_FIELDS which is not released.
But if you actually want to use this one, look thru SE37 at parameter definition, so use structure DBFIELD...
Are you trying to get information on a DB table not defined in ddic, or looking for difference between DDIC and database (as DB02 and similar transactions) ?
Regards,
Raymond