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AmplifierProblem-Slide4

Another important term is the CMRR or Common mode rejection ratio. It specifies how well the in-amp gets ride of the common mode signal at its input. This is a key specification for an in-amp. No instrumentation amplifier des this perfectly, but ADI in-amps have pretty high performance in this aspect. The ADI integrated in-amp exceeds the CMRR performance of any approach using 3-discrete op-amps by a several orders of magnitude. On average, if the engineer requires a CMRR of greater than 60dB, an integrated amp typically needs to be purchased. In the examples shown here, an amp with bad CMR allows the common mode signals to appear at the output with little attenuation whereas an amp with good CMR reduces the common mode signal to a very small amount at the output, which is what is desired.

PTM Published on: 2010-12-16