+

RL

OPA340

ADC

R

C

IS

Buffer amplifier

drives ADC without

affecting gain

INA138-Q1

or

INA168-Q1

VIN+

VIN±

RS

OUT

GND

12

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

8.2 Typical Applications

The INA1x8-Q1 are designed for current-shunt measurement circuits (see Figure 9) but its basic function is

useful in a wide range of circuitry. With a little creativity, many unforeseen uses can be found in measurement

and level-shifting circuits. A few ideas are illustrated in the following subsections.

8.2.1 Buffering Output to Drive an ADC

Digitize the output of the INA138-Q1 or INA168-Q1 devices using a 1-MSPS analog-to-digital converter (ADC).

Figure 10. Buffering Output to Drive an ADC

8.2.1.1 Design Requirements

For this design example, use the input parameters shown in Table 2.

Table 2. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Supply voltage, V+

5 V

Common-mode voltage, VCM

INA138-Q1: 2.7 V to 36 V

INA168-Q1: 2.7 V to 60 V

Full-scale shunt voltage, VSENSE

50 mV to 100 mV

Load resistor, RL

5 kΩ to 500 kΩ

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Selecting RS and RL

In Figure 10, the value chosen for the shunt resistor, RS, depends on the application and is a compromise

between small-signal accuracy and maximum permissible voltage loss in the measurement line. High values of

RS provide better accuracy at lower currents by minimizing the effects of offset, while low values of RS minimize

voltage loss in the supply line. For most applications, best performance is attained with an RS value that provides

a full-scale shunt voltage range of 50 mV to 100 mV. Maximum input voltage for accurate measurements is 500

mV.

Choose an RL that provides the desired full-scale output voltage. The output impedance of the INA1x8-Q1 OUT

pin is very high, permitting the use of RL values up to 500 kΩ with excellent accuracy. The input impedance of

any additional circuitry at the output must be much higher than the value of RL to avoid degrading accuracy.

Some ADCs have input impedances that significantly affects measurement gain. The input impedance of the

ADC can be included as part of the effective RL if the ADC input can be modeled as a resistor to ground.

Alternatively, an op amp can be used to buffer the ADC input, as shown in Figure 10. The INA1x8-Q1 are current

output devices, and as such, have an inherently large output impedance. The output currents from the amplifier

are converted to an output voltage using the load resistor, RL, connected from the amplifier output to ground. The

ratio of the load resistor value to that of the internal resistor value determines the voltage gain of the system.