Important clarification post from Google: Search Quality Score is independent of Content Quality Score. The first, combined with an advertiser's max bid, determines Adwords rank. The second relates to quality of a page serving AdSense ads for Google content ads.
Does your content Quality Score impact your search Quality Score?
Since this is especially important, we want to make sure that everyone sees the answer to this question, and that answer is 'No.' Performance on the content network will not impact your search Quality Score, and vice versa.
Clarification post here
More from Google about landing page quality considerations here