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Current sensing is vital to system reliability. Here’s how to do it

October 4, 2016

currentsenseamplifiers

As electronic systems pack higher performance into smaller volumes, thermal protection is becoming a thornier problem: A rise in the temperature of a semiconductor junction affects various physical properties, degrading the performance of a device and, ultimately, causing the system to fail.

Since junction temperature is a function of the current passing through it, sensing current is the best way to spot – and prevent — potential problems.

While a fuse can provide simple and cheap overcurrent protection, it’s a binary device: Open when current exceeds the fuse’s limit and closed until then. Once blown it has to be replaced.

In contrast, continuous current measurements can be part of a feedback loop, not only to adjust system performance for safe operation, but also to increase efficiency; the rotational speed and direction of a motor, for example, are reflected in the magnitude and direction of the current.

There are two techniques for current sensing: Invasive and non-invasive.

Alternating currents can be sensed non-invasively by a coil – in the proximity of the current-carrying conductor — across which the magnetic field induces a voltage proportional to the current. Precise direct current sensing uses a differential amplifier to measure the voltage across a shunt resistor in series with the load. It’s an invasive measurement since the shunt resistor and sensing circuitry are electrically connected to the monitored system. Also, the shunt resistor dissipates power, which needs to be minimized.

Using a standard op amp as a differential amplifier requires expensive external precision resistors. Instead, a specialized current-sense amplifier that integrates the op amp and a precisely matched resistive gain-setting network enables a more accurate measurement, over a wide temperature range, with a smaller footprint.

More on integrated current sensors and their use in automotive and industrial applications can be found in this concise guide to current sense amplifiers.