ZHCSC46C February 2014 – June 2021 INA300
The device measures the differential voltage developed across a resistor when current flows through it to determine if the monitored current exceeds a defined limit. This resistor is referred to as a current-sensing resistor or a current-shunt resistor, with each term used interchangeably. The flexible design of the device allows for measuring a wide differential input signal range across this current-sensing resistor, which can extend up to 250 mV.
Selecting the value of this current-sensing resistor is based primarily on two factors: the required accuracy of the current measurement and the allowable power dissipation across the current-sensing resistor. Larger voltages developed across this resistor allow more accurate measurements. This large signal accuracy improvement results from the fixed internal amplifier errors that are dominated by the inherent input offset voltage of the device. When the input signal decreases, these fixed internal amplifier errors become a larger portion of the measurement and increase the uncertainty in the measurement accuracy. When the input signal increases, the measurement uncertainty is reduced because the fixed errors are a smaller percentage of measured signal.
A system design trade-off for improving the measurement accuracy using larger input signals is the increase in power across the current-sensing resistor. Increasing the value of the current-shunt resistor increases the differential voltage developed across the resistor when current passes through the component. This increase in voltage across the resistor increases the power that the resistor must be able to dissipate. Decreasing the value of the current-shunt resistor value reduces the power dissipation requirements of the resistor, but increases the measurement errors resulting from the decreased input signal. Selecting the optimal value for the shunt resistor requires factoring both the accuracy requirement for the specific application and the allowable power dissipation of this component.
An increasing number of low ohmic-value resistors are becoming available with values as low as 200 µΩ, with power dissipations of up to 5 W that enable large currents to be monitored with sensing resistors.