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INA169 Datasheet(PDF) 11 Page - Texas Instruments |
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INA169 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 25 page I S OPA340 INA139 3 4 Z IN R L 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 Selecting RS and RL In Figure 9 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 of 50 mV to 100 mV; maximum input voltage for accurate measurements is 500 mV. RL is chosen to provide the desired full-scale output voltage. The output impedance of the INA139 and INA169 OUT terminal is very high, which permits using values of RL up to 100 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 Analog-to-Digital converters (ADC) have input impedances that will 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 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, 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 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 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 INA139 or INA169 devices and the input to the ADC. Copyright © 2000–2015, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: INA139 INA169 |
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