INA4181A1IPWR.B datasheet(24/62 Pages) TI2

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8.1.2 RSENSE and Device Gain Selection

The accuracy of the INAx181 is maximized by choosing the current-sense resistor to be as large as possible.

A large sense resistor maximizes the differential input signal for a given amount of current flow and reduces

the error contribution of the offset voltage. However, there are practical limits as to how large the current-sense

resistor can be in a given application. The INAx181 have typical input bias currents of 75 µA for each input

when operated at a 12-V common-mode voltage input. When large current-sense resistors are used, these

bias currents cause increased offset error and reduced common-mode rejection. Therefore, using current-sense

resistors larger than a few ohms is generally not recommended for applications that require current-monitoring

accuracy. Another common restriction on the value of the current-sense resistor is the maximum allowable

power dissipation that is budgeted for the resistor. Equation 2 gives the maximum value for the current sense

resistor for a given power dissipation budget:

MAX

SENSE

MAX

(2)

where:

• PDMAX is the maximum allowable power dissipation in RSENSE.

• IMAX is the maximum current that flows through RSENSE.

An additional limitation on the size of the current-sense resistor and device gain is due to the power-supply

voltage, VS, and device swing to rail limitations. To make sure that the current-sense signal is properly passed

to the output, both positive and negative output swing limitations must be examined. Equation 3 provides the

maximum values of RSENSE and GAIN to keep the device from hitting the positive swing limitation.

MAX

SENSE

REF

GAIN

(3)

where:

• IMAX is the maximum current that flows through RSENSE.

• GAIN is the gain of the current sense-amplifier.

• VSP is the positive output swing specified in the data sheet.

• VREF is the externally applied voltage on the REF pin.

To avoid positive output swing limitations when selecting the value of RSENSE, there is always a trade-off

between the value of the sense resistor and the gain of the device to consider. If the sense resistor selected for

the maximum power dissipation is too large, then selecting a lower-gain device to avoid positive swing limitations

is possible.

The negative swing limitation places a limit on how small of a sense resistor can be used in a given application.

Equation 4 provides the limit on the minimum size of the sense resistor.

MIN

SENSE

REF

GAIN > V

(4)

where:

• IMIN is the minimum current that flows through RSENSE.

• GAIN is the gain of the current sense amplifier.

• VSN is the negative output swing of the device (see Rail-to-Rail Output Swing).

• VREF is the externally applied voltage on the REF pin.

In addition to adjusting the offset and gain, the voltage applied to the REF pin can be slightly increased to avoid

negative swing limitations.

INA181, INA2181, INA4181

SBOS793H – APRIL 2017 – REVISED NOVEMBER 2023

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