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INA138-Q1 Datasheet(PDF) 6 Page - Texas Instruments |
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INA138-Q1 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 22 page ![]() INA138-Q1, INA168-Q1 SGLS174G – SEPTEMBER 2003 – REVISED JANUARY 2013 www.ti.com APPLICATION INFORMATION Figure 9 illustrates the basic circuit diagram for both the INA138-Q1 and INA168-Q1. Load current IS is drawn from supply VS through shunt resistor RS. The voltage drop VS 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 to a voltage, VOUT, at the OUT pin. The transfer function for the INA138-Q1 is: IO = gm (VIN+ − VIN−) where gm = 200 μA/V. In the circuit of Figure 9, the input voltage, (VIN+ − VIN−), is equal to IS × RS and the output voltage, VOUT, is equal to IO × RL. The transconductance, gm, of the INA138-Q1 is 200 μA/V. The complete transfer function for the current measurement amplifier in this application is: VOUT = (IS) (RS) (200 μA/V) (RL) The maximum differential input voltage for accurate measurements is 0.5 V, which produces a 10 μA output current. A differential input voltage of up to 2 V does not cause damage. Differential measurements (pins 3 and 4) must be unipolar with a more-positive voltage applied to pin 3. If a more-negative voltage is applied to pin 3, the output current, IO, is zero, but it does not cause damage. BASIC CONNECTION Figure 9 shows the basic connection of the INA138-Q1. The input pins, VIN+ and VIN−, should be connected as closely as possible to the shunt resistor to minimize any resistance in series with the shunt resistance. The output resistor, RL, is shown connected between pin 1 and ground. Best accuracy is achieved with the output voltage measured directly across RL. This is especially important in high-current systems where load current could flow in the ground connections, affecting the measurement accuracy. No power-supply bypass capacitors are required for stability of the INA138-Q1. However, applications with noisy or high-impedance power supplies may require decoupling capacitors to reject power-supply noise. Connect bypass capacitors close to the device pins. POWER SUPPLIES The input circuitry of the INA138-Q1 can accurately measure beyond its power-supply voltage, V+. For example, the V+ power supply can be 5 V, whereas the load power supply voltage is up to 36 V (or 60 V with the INA168- Q1). The output voltage range of the OUT terminal, however, is limited by the lesser of the two voltages (see Output Voltage Range section). SELECTING RS AND RL 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, the 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. RL is chosen to provide the desired full-scale output voltage. The output impedance of the INA138-Q1 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 (A/D) converters have input impedances that significantly affects measurement gain. The input impedance of the A/D converter 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 A/D converter input. Figure 9 shows the recommended values of RL. 6 Submit Documentation Feedback Copyright © 2003–2013, Texas Instruments Incorporated Product Folder Links: INA138-Q1 INA168-Q1 |
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