+

RL

OPA340

ADC

RS

C

IS

Buffer amplifier

drives ADC without

affecting gain

INA138

or

INA168

VIN+

VIN-

11

INA138, INA168

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SBOS122E – DECEMBER 1999 – REVISED DECEMBER 2017

Product Folder Links: INA138 INA168

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Copyright © 1999–2017, Texas Instruments Incorporated

8.2 Typical Applications

The INA1x8 devices are designed for current shunt measurement circuits, as shown in Figure 9, but basic device

function is useful in a wide range of circuitry. A creative engineer will find many unforeseen uses in measurement

and level-shifting circuits. A few ideas are illustrated in Figure 10 through Figure 18.

8.2.1 Buffering Output to Drive an ADC

Figure 10. Buffering Output to Drive an ADC

8.2.1.1 Design Requirements

Digitize the output of the INA1x8 devices using a 1-MSPS analog-to-digital converter (ADC).

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

Selecting the Shunt Resistor and RL

In Figure 9 the value chosen for the shunt resistor depends on the application and is a compromise between

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

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

resistor minimize voltage loss in the supply line. For most applications, best performance is attained with a shunt

resistor value that provides a full-scale shunt voltage range of 50 mV to 100 mV. Maximum input voltage for

accurate measurements is 500 mV.

The load resistor, RL, is chosen to provide the desired full-scale output voltage. The output impedance of the

INA1x8 OUT terminal is very high which permits using values of RL up to 500 kΩ with excellent accuracy. The

input impedance of any additional circuitry at the output should be much higher than the value of RL to avoid

degrading accuracy.

Some analog-to-digital converters (ADCs) have input impedances that significantly affect measurement gain. The

input impedance of the ADC can be included as part of the effective RL if its input can be modeled as a resistor

to ground. Alternatively, an op amp can be used to buffer the ADC input. The INA1x8 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 via 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.

In many applications digitizing the output of the INA1x8 device is required, and can be accomplished by

connecting the output of the amplifier to an ADC. It is very common for an ADC to have a dynamic input

impedance. If the INA1x8 output is connected directly to an ADC input, the input impedance of the ADC is

effectively connected in parallel with the gain setting resistor RL. This parallel impedance combination will affect

the gain of the system and the impact on the gain is difficult to estimate accurately. A simple solution that

eliminates the paralleling of impedances, simplifying the gain of the circuit is to place a buffer amplifier, such as

the OPA340, between the output of the INA138 or INA168 device and the input to the ADC.