There are times when a small signal needs to be measured in the presence of a large common-mode signal. Traditional instrumentation amplifiers (in-amps) that have two op amp or three op amp internal configurations are commonly used in these applications. Although in-amps have good common-mode rejection ratios (CMRR), price and sometimes specifications prevent their usage in these applications. In-amps may not have the right bandwidth, dc accuracy, or power consumption requirements that the user requires. Therefore, in these situations, users build their own difference amplifiers by using a single amplifier and external resistors as an alternative to instrumentation amplifiers. Unless a set of tightly matched resistors is used, CMRR of these circuits are very low. This application note presents several ways to build and optimize the performance of a discrete difference amplifier. It also recommends amplifiers that make the overall solution cost/performance competitive with monolithic instrument amplifiers.
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