To deal with the replaced equipment in power system, a resource reused solution is proposed. And a merging unit of analog input is taken for instance, which is converted to AC sampling test device by software and hardware adjustments. First of all, the present resources of the merging unit are analyzed, while the hardware adjusting scheme with simplified configuration is given, compatible with the transformers, modules, devices and other different sampling access modes. Then according to the application requirements, the software adjusting scheme is introduced, adopting incremental mode and adding AC sampling test functions. Additionally the principles of different testing function are analyzed, such as decay time constant, maximum peak instantaneous error and waveform monitoring, while the detailed design scheme and implementation steps are discussed. Finally, the specific application instances are given according to different test occasions. The results show that the device can be applied to AC sampling test.