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INA138-Q1, INA168-Q1

SGLS174J – SEPTEMBER 2003 – REVISED AUGUST 2018

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Product Folder Links: INA138-Q1 INA168-Q1

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Copyright © 2003–2018, Texas Instruments Incorporated

7.3 Feature Description

7.3.1 Output Voltage Range

The output of the INA1x8-Q1 is a current that is converted to a voltage by the load resistor, RL. The output

current remains accurate within the compliance voltage range of the output circuitry. The shunt voltage and the

input common-mode and power-supply voltages limit the maximum possible output swing. The maximum output

voltage (Vout max) compliance is limited by either Equation 1 or Equation 2, whichever is lower:

Vout max = (V+) – 0.7 V − (VIN+ – VIN−)

(1)

or

Vout max = VIN– − 0.5 V

(2)

7.3.2 Bandwidth

Measurement bandwidth is affected by the value of the load resistor, RL. High gain produced by high values of

RL yields a narrower measurement bandwidth (see the Typical Characteristics section). For the widest possible

bandwidth, keep the capacitive load on the output to a minimum. Reduction in bandwidth due to capacitive load

is shown in the Typical Characteristics section.

If bandwidth limiting (filtering) is desired, add a capacitor to the output (see Figure 12). This capacitor does not

cause instability.

7.4 Device Functional Modes

For proper operation, the INA1x8-Q1 must operate within the specified limits. Operating either device outside of

their specified power-supply voltage range, or their specified common-mode range, results in unexpected

behavior, and is not recommended. Additionally, operating the output beyond the specified limits with respect to

power-supply voltage and input common-mode voltage also produces unexpected results. See the Electrical

Characteristics section for the device specifications.

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-Q1 and INA168-Q1. Load current IS is drawn

from supply VP through shunt resistor RS. The voltage drop in the shunt resistor 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,

IO, to a voltage, VOUT, at the OUT pin. The transfer function for the INA1x8-Q1 is shown in Equation 3:

IO = gm (VIN+ − VIN−)

where

•

gm = 200 μA/V

(3)

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

IO × RL. The transconductance, gm, of the INA1x8-Q1 is 200 μA/V. The complete transfer function for the current

measurement amplifier in this application is shown in Equation 4:

VOUT = (IS) (RS) (200 μA/V) (RL)

(4)

The maximum differential input voltage for accurate measurements is 0.5 V, producing a 100-μA output current.

A differential input voltage of up to 2 V does not cause damage. Differential measurements (VIN+ and VIN− pins)

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

VIN+ pin, IO goes to zero, but no damage occurs.