A memory system includes a memory module and a memory controller. The memory module includes a plurality of memory devices with corresponding ZQ calibration circuits therein. The memory controller, which is electrically coupled to the memory module, includes a ZQ global managing circuit therein. This ZQ global managing circuit is configured to determine a plurality of calibration values associated the corresponding ZQ calibration circuits in the plurality of memory devices, in response to calibration result data generated by the plurality of ZQ calibration circuits. The memory module is mounted within a memory slot. In addition, the plurality of calibration values account for signal loading characteristics of the memory module within the memory slot.