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How current sense amplifiers can replace high common mode instrumentation amplifiers.

Hardware design
August 06, 2020 by David 1462

Instrumentation amplifiers (IA) are often used in occasions that require high gain accuracy and high DC accuracy, such as test and measurement and experimental instruments, but such devices are relatively expensive. The current-sense amplifier is cheap and can handle higher common-mode voltages. Some features are similar to instrumentation amplifiers. How to replace instrumentation amplifiers with current-sense amplifiers in -48V to +5V power converters?

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Nevaeh Posted on August 6, 2020

The current-sense amplifier detects the voltage across the precision "current-sense resistor" connected to its differential input, and the resistor is located at a higher potential than other circuits in the system. The amplifier input is a differential voltage, and the output is a single-ended signal and is precisely proportional to the differential input. This signal is referenced to the system ground.

For a given current, considering the precision gain of the current-sense amplifier, only a small voltage difference is needed across the current-sense resistor to generate the corresponding output measurement voltage, thereby reducing the current-sense power consumption on the power line. It can be seen that the current-sense amplifier conforms to the basic definition of a voltage-type instrumentation amplifier: a precision differential amplifier.

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Skylar Posted on August 6, 2020

The advantage of the current detection amplifier is that it can accurately amplify the signal. Therefore, it can not only detect the current, but also separate the signal from the high voltage. The following figure shows the standard application

48V-+5V power converter, common ground.

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Ryleigh Posted on August 6, 2020

In terms of concept, it is not difficult to design a switching power converter with opposite input voltage and output voltage polarity. However, considering the engineering details, the choice of circuit topology is more difficult. For a converter working on a positive power supply, the output voltage and the reference level of the feedback voltage are the same, and both are the negative end of the power supply, here is -48V defined by the positive power converter topology.

For applications where the output positive voltage must be in the same ground with the input voltage, the -48V reference point conflicts with the application requirements. The input voltage and output voltage of isolated topologies (such as flyback and forward) are isolated from each other, and their reference points are different, so they can meet this type of application. Despite the high cost and complex circuit, they are still widely used.

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