Significant growth in demand for mixed signal parts, and an increased level of integration of such parts into SOCs have created or exacerbated several test related challenges. Techniques like design-for-testability (DFT) have been applied to overcome some of these challenges. In this paper, we present case studies of one of the most common DFT techniques for mixed-signal devices, namely, analog loopback testing of data converters. Theoretical analysis of loopback testing is presented, along with silicon test results for stand-alone, internal loopback and external loopback testing of data converter in an industrial chip. These results are used to evaluate the feasibility and effectiveness of these two variants of mixed-signal DFT, i.e., internal and external loopback testing of data converters.