How to Increase the Common-Mode Rejection Ratio of Differential Amplifiers
Differential amplifier circuits are required in the analog domain for a wide variety of applications (such as, in measurement technology, where extreme accuracy is required). In addition to high-precision operational amplifiers, matched resistors for the circuitry are also decisive for this accuracy, because any additional deviation between their ratios leads to an undesired common-mode error. Common Mode Rejection Ratio (CMRR) is therefore an important parameter in such circuits.
Conventional resistors do not have constant values in practice. They are subjected to mechanical loads and the effects of temperature. Depending on the requirement, resistors with different tolerances or matched resistor pairs or networks may be used. With these matched resistor networks, the overall CMRR of the amplifier circuit can be dramatically improved.
This paper shows how to carefully select the external resistor circuit based on the requirements for accuracy of the differential amplifier circuit, for a high system performance. Alternatively, integrated differential amplifiers, like the LTC6363, can be used. These have integrated matched resistors on the chip.
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