If you are using the 'questionnaire' activity as your feedback tool you can go to Questionnaire Administration|Edit Settings and set the Submission Grade to whatever grade you are giving for the feedback;I use 100% (see Figure 1 below). Then set one of the Certificate Grade condition criteria to the questionnaire, and set the minimum grade to less than the Submission Grade; in my case I set it to 99.9% (See Figure 2 below. Hope this helps.
Figure 1 Example of Submission Grade setup in Questionnaire
Figure 2 Example of Grade Condition set up in Certificate