8.1.2 RSENSE and Device Gain Selection

The accuracy of any current-sense amplifier 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 because of the resistor size and maximum allowable power

dissipation. Equation 2 gives the maximum value for the current-sense resistor for a given power dissipation

budget:

MAX

SENSE

2

MAX

PD

R

I

(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. In order 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

SP

REF

I

R

GAIN < V

V

u

u

(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 as specified in the data sheet.

• VREF is the reference input. This is node is internally grounded for the INA191 and a value of 0 V should be

used for that device.

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 under consideration. If the sense resistor

selected for the maximum power dissipation is too large, then it is possible to select a lower-gain device in order

to avoid positive swing limitations.

The zero current output voltage 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.

I

MIN × RSENSE × GAIN > VZL - VREF

(4)

where:

• IMIN is the minimum current flows through RSENSE.

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

• VZL is the zero current output voltage of the device (see the Section 7.3.7 section for more information).

• VREF is the reference input. This node is internally grounded for the INA191 and a value of 0 V should be

used for that device.

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INA191, INA2191

SLYS020C – FEBRUARY 2019 – REVISED AUGUST 2021

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