Shunt Resistor Selection for High-Accuracy, Optically Isolated Sigma-Delta Modulator Current-Sensing Solution

As modern factory automation (including servo motors, industrial robots, and computer numerical control [CNC] machines) trends toward higher precision, power, speed, multi-axis, and multi-direction, the requirement for accurate sensing of motor phase current has become ever more demanding. Traditionally, current sensing is performed by a current transformer (CT) or Hall Effect Sensor (HES), but these solutions are bulky, expensive, and less accurate over operating temperatures. A smaller, low-cost solution can be easily realized by connecting a shunt resistor directly to the sigma-delta modulator. Phase current flows through a shunt resistor with the resistance value selected such that the maximum current range corresponds to an optimum recommended low-input voltage range of the sigma-delta modulator. At this low voltage, power dissipation loss across the shunt resistor is minimized.
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