Current sense amplifier

Current sense amplifier

Current sense amplifiers (also called current shunt amplifiers) are special purpose amplifiers that output a voltage proportional to the current flowing in a power rail. They utilize a "current-sense resistor" to convert the load current in the power rail to a small voltage, which is then amplified by the current-sense amplifiers. The currents in the power rail can be in the range of 1 A to 20 A, as a result, the current-sense resistor is a very low ohmic value resistor (usually in the range of a 1 mΩ to 100 mΩ). These amplifiers are designed to amplify a very small "sense voltage"—in the order of 10 mV to 100 mV in the presence of very large common-mode voltages—in the order of 5 V to 30 V). DC precision (low input offset voltage) and high common-mode rejection ratio (CMRR) are distinguishing characteristics of these amplifiers. Current sense amplifiers can either measure current flowing in a single direction or bidirectional amplifiers can measure current flow in both directions through the sense resistor.[1]

Traditional differential amplifiers (same applies to operational amplifiers or op amps) powered between two power supply rails (say VCC and VEE) can only process/amplify signals that lie between these two power rails. If any voltage greater than these power supply rails are applied to their input pins, internal ESD protection diodes turn-on, causing large currents to flow and damage these parts

High-side current-sense amplifiers, by contrast, are amplifiers with a special architecture. Even when powered from a low-voltage power rail such as VCC = 5 V and VEE = 0 V, they can withstand pin voltages much higher than VCC and much lower than VEE. These amplifiers use specialized ESD structures that enable them to have this functionality. In addition, their input stages are designed such that when the input common-mode voltage is much higher than VCC (or conversely, much lower than VEE), the input amplifier stage powers itself from the input common-mode voltage instead of VCC (or VEE).[2]

Examples include LTC6101,MAX4080, AD8210 and INA193. In special cases, no VCC pin is required to accomplish current-sensing—such as MAX9938.

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