US Patent 7852111 Semiconductor device and impedance adjustment method of the same

A 4-bit counter outputs a 4-bit counted value CNTp based on an up-and-down signal Sp supplied from a comparator. A weighting selection circuit performs weighting based on a deviation from an average value of the DC characteristic of each PMOS transistor, and assigns a transistor having the smallest deviation to Bit 1 (LSB) of the 4-bit counter. The weighting selection circuit assigns two PMOS transistors to Bit 2 of the 4-bit counter, four PMOS transistors to Bit 3, and eight PMOS transistors to Bit 4 (MSB). Then, the weighting selection circuit selects transistors P3-1 to P3-30 based on the counted value CNTp output from the 4-bit counter.