VOLTAGE GAIN

EXACT R ( )

L

NEAREST 1% R ( )

L

1

2

5

10

20

50

100

5k

10k

25k

50k

100k

250k

500k

4.99k

10k

24.9k

49.9k

100k

249k

499k

I

V

P

Load Power Supply

Shunt

R

S

S

V+ power can be common or

independent of load supply.

V+

V

IN+

V

IN–

3

4

Load

R

G1

5 k

R

G2

5 k

5

Q1

INA138

2

OUT

1

I

0

+

R

V

L

O

–

2.7 V to 36 V

(1)

2.7 V

(V+)

36 V

(1)

≤

≤

Copyright © 1999, Texas Instruments Incorporated

10

INA138, INA168

SBOS122E – DECEMBER 1999 – REVISED DECEMBER 2017

www.ti.com

Product Folder Links: INA138 INA168

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Copyright © 1999–2017, Texas Instruments Incorporated

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

8.1.1 Operation

Figure 9 illustrates the basic circuit diagram for both the INA138 and INA168 devices. Load current IS is drawn

from supply VS through shunt resistor RS. The voltage drop in shunt resistor VS is forced across RG1 by the

internal op amp, causing current to flow into the collector of Q1. External resistor RL converts the output current

to a voltage, VOUT, at the OUT pin. The transfer function for the INA138 device is:

IO = gm(VIN+ – VIN–)

(3)

where gm = 200 µA/V.

In the circuit of Figure 9, the input voltage, (VIN+ – VIN–), is equal to IS × RS and the output voltage, VOUT, is equal

to IO × RL. The transconductance, gm, of the INA138 device is 200 µA/V. The complete transfer function for the

current measurement amplifier in this application is:

VOUT = (IS) (RS) (200 µA/V) (RL)

(4)

The maximum differential input voltage for accurate measurements is 0.5 V, which produces a 100-µA output

current. A differential input voltage of up to 2 V will not cause damage. Differential measurements (pins 3 and 4)

must be unipolar with a more-positive voltage applied to pin 3. If a more-negative voltage is applied to pin 3, the

output current, IO, will be zero, but it will not cause damage.

(1)

Maximum VP and V+ voltage is 60 V with INA168.

Figure 9. Basic Circuit Connections