The use of current sense resistors, such as Microhm Electronics shunt resistors,  is part of a trend in motor control system design that benefits from adopting new digital isolation tech­nologies. These technologies offer higher reliability levels to designers based on the introduction of the component level standard IEC 60747-17, which specifies the performance, test, and certification requirements for capacitive and magnetically coupled isolators.


Digital control of a motor drive incorporates current sense resistors for sensing winding current. The article offers recommendations for selecting the best current sense resistor for this application.

Given that the output of the AD7403 is a 16-bit number, the potential accuracy of the current measurement is limited not by the ADC conversion, but by the voltage reading itself. The drift of the resistance with temperature will vary depending on the material used in the resistor element, as well as the power rating and the actual physical size of the component.


Resistive elements made up of special alloys of nickel, copper, and manganese have parabolic resistance drift curves, these alloys are the most accurate materials used for current sensing applications. Microhm MMS4026 is such kind a shunt resistor.

Let us use Microhm Electronics' part number MMS4026L500F for calculating the resistance drift at full power and at an ambient temperature of 70°C. MMS4026L500F is a 0.5 mΩ (±1%) sense resistor rated to 5 watts of power, at a maximum ambient temperature of 130°C. It derates from 100% power to 0 W at 170°C. The thermal resistance of the component therefore, is 8°C/W. At full power and an ambient temperature of 70°C, we can expect the surface temperature of the component to reach 110°C.