8.2 Typical Application

8.2.1 Microamp Current Measurement

The low input bias current of the INAx191 provides accurate monitoring of small-value currents. To accurately

monitor currents in the microamp range, increase the value of the sense resistor to increase the sense voltage

so that the error introduced by the offset voltage is small. The circuit configuration to monitor low-value currents

is shown in Figure 8-6. As a result of the differential input impedance of the INAx191, limit the value of RSENSE to

1 kΩ or less for best accuracy.

ENABLE

VS

OUT

REF

(1)

GND

IN+

IN±

LOAD

0.1 F

5 V

12 V

RSENSE ” 1 kO

INA191

INA2191 (½)

(1) REF pin only available on INA2191

Figure 8-6. Measuring Microamp Currents

8.2.1.1 Design Requirements

The design requirements for the circuit shown in Figure 8-6, are listed in Table 8-1

Table 8-1. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Power-supply voltage (VS)

5 V

Bus supply rail (VCM)

12 V

Minimum sense current (IMIN)

1 µA

Maximum sense current (IMAX)

150 µA

Device gain (GAIN)

25 V/V

Unidirectional Application

VREF = 0 V

8.2.1.2 Detailed Design Procedure

The maximum value of the current-sense resistor is calculated based on choice of gain, value of the maximum

current the be sensed (IMAX), and the power supply voltage (VS). When operating at the maximum current, the

output voltage must not exceed the positive output swing specification, VSP. For the given design parameters,

the maximum value for RSENSE calculated in Equation 6 is 1.321 kΩ.

SP

SENSE

MAX

V

R

<

I

GAIN

u

(6)

However, because this value exceeds the maximum recommended value for RSENSE, a resistance value of 1

kΩ must be used. When operating at the minimum current value, IMIN the output voltage must be greater than

the swing to GND (VSN), specification. For this example, the output voltage at the minimum current (VOUTMIN)

calculated in Equation 7 is 25 mV, which is greater than the value for VSN.

OUTMIN

MIN

SENSE

V

I

R

GAIN

u

u

(7)

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INA191, INA2191

SLYS020C – FEBRUARY 2019 – REVISED AUGUST 2021

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