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OPA340

INA139

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IN

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Buffer of amp drives the A/D converter

without affecting gain.

INA139, INA169

www.ti.com

SBOS181E – DECEMBER 2000 – REVISED DECEMBER 2015

8.2 Typical Applications

The INA139 is designed for current shunt measurement circuits, as shown in Figure 9, but its basic 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 14 through Figure 18.

8.2.1 Buffering Output to Drive an ADC

Figure 10. Buffering Output to Drive the A/D Converter

8.2.1.1 Design Requirements

Digitize the output of the INA139 or INA169 devices using a 1-MSPS analog-to-digital converter (ADC).

8.2.1.2 Detailed Design Procedure

8.2.1.2.1

shunt voltage of 50 mV to 100 mV; maximum input voltage for accurate measurements is 500 mV.

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

accuracy.

Some Analog-to-Digital converters (ADC) have input impedances that will significantly affect measurement gain.

resistor to ground. Alternatively, an operational amplifier can be used to buffer the ADC input, as shown in

Figure 10. The INA139 and INA169 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 through the load resistor,

resistor value determines the voltage gain of the system.

In many applications digitizing the output of the INA139 or INA169 devices is required. This is 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 INA139 or INA169 output is connected directly to an ADC input, the input impedance of the

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 INA139 or INA169 devices and the input to the ADC.

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