Instead, you need to have each ad go to a separate URL so you can track them separately with your site logs. The separate URLs can differ simply by a "GET" parameter that a script can read and store in a cookie, which then can be added to the record created when a conversion is made.
I'd suggest playing with the custom reports while including parts of ads and conversions to learn how to pull up the report you want. It's in there, just have to determine exactly the data you want to get.