A memory device includes memory cell array including a plurality of memory cells that store data, a first transmitter that transmits the data to an external device through a first data line, and a ZQ controller that performs a ZQ calibration operation to generate a first ZQ code for impedance matching of the first data line. The first transmitter encodes the first ZQ code and the first data based on a first clock and drives the first data line based on the encoded result based on a second clock.