US Patent 7986150 Calibration circuit

To include a first replica buffer that has substantially the same circuit configuration as a pull-up circuit which constitutes an output buffer and a second replica buffer that has substantially the same circuit configuration as a pull-down circuit which constitutes the output buffer. When a first calibration command ZQCS is issued, both a control signal ACT1 and ACT2 is activated, and a calibration operation is performed for both the first replica buffer and the second replica buffer in parallel.