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INA300

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SBOS613B – FEBRUARY 2014 – REVISED APRIL 2016

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7.3 Feature Description

7.3.1 Selecting a Current-Sensing Resistor

The device measures the differential voltage developed across a resistor when current flows through it to

determine if the current being monitored exceeds a defined limit. This resistor is commonly referred to as a

current-sensing resistor or a current-shunt resistor, with each term commonly used interchangeably. The flexible

design of the device allows for measuring a wide differential input signal range across this current-sensing

resistor, extending 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 to be made. 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 the signal being

measured.

A system design trade-off for improving the measurement accuracy through the use of 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 very low ohmic-value resistors are becoming available with values reaching down to

200 µ

Ω with power dissipations of up to 5 W that enable very large currents to be accurately monitored with

sensing resistors.

7.3.1.1 Selecting a Current-Sensing Resistor: Example

In this example, the trade-offs involved in selecting a current-sensing resistor are discussed. This example

requires a 5% measurement accuracy for detecting a 10-A overcurrent event at a 50-µs delay setting where only

250 mW is allowable for the dissipation across the current-sensing resistor at the full-scale current level.

Although the maximum power dissipation is defined as 250 mW, a lower dissipation is preferred to improve

system efficiency. Some initial assumptions are made that are used in this example: the limit setting resistor,

source, 0.5%, is used. Given the total error budget of 5%, up to 3.5% of error is available to be attributed to the

internal offset of the device.