Digital signature mendetaory fo 44AD

is there any audit under 44AD? as audit is refered under s/44AB, and every audit case is subject to digital signature w.e.f 1st july 11

No.. there is no requirement for Digital Signature :)

As per 44AD(5), an assessee who claims that his  profits and gains from the eligible business are lower than the profits and gains specified U/s 44AD(1), he should maintain books and should get his accounts audited U/s 44AB and hence it becomes audit U/s44AB, which makes digital signature mandatory to file return of income of such an assessee.