Question: why cmrr is measured in db?
Answer: Mathematically it is the ratio of Differential Gain (AD) by Common mode gain (AC). CMRR is also known as figure of merit. Common Mode input is applied to both the terminals of op-amp and then the output Voc is measured. The CMRR is generally expressed in db and for op-amp 741 IC,it's value is 90 db.