ALERT

CMPREF

CMPIN, VOUT

4 V

Vs

4 V

VOUT

RESET

ALERT

VS+

GND

CMPIN

CMPREF

10 k:

+

-

+

-

G = 20, 50,

100, 200

5 V

2.7 V to 5.5 V

INA381

REF3140

4 V

0.2% , 15ppm/°C

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12

INA381

SBOS848 – DECEMBER 2017

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Figure 5. External Reference Voltage

7.3.7 Selecting a Current-Sensing Resistor

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

component 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.

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 for more accurate measurements to be made. Amplifiers have fixed internal

errors that are largely dominated by the inherent input offset voltage. 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. Therefore, the use of larger-value, current-

sensing resistors inherently improves the measurement accuracy.

However, a system design trade-off must be evaluated through use of larger input signals for improving the

measurement accuracy. Increasing the current-sense resistor value results in increased power dissipation 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 more widely available with values reaching

down as low as 1 mΩ or lower with power dissipations of up to 5 W that enable large currents to be accurately

monitored with sensing resistors.