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INA138 Datasheet(PDF) 5 Page - Burr-Brown (TI) |
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INA138 Datasheet(HTML) 5 Page - Burr-Brown (TI) |
5 / 15 page ![]() INA138, INA168 5 SBOS122C www.ti.com TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, V+ = 5V, VIN+ = 12V, and RL = 125kΩ, unless otherwise noted. STEP RESPONSE 10 µs/div 200mV G = 1 100mV 100mV G = 1 0mV 50mV/div STEP RESPONSE 10 µs/div G = 25 0V G = 10 0V 1V/div 500mV/div OPERATION Figure 1 illustrates the basic circuit diagram for both the INA138 and INA168. Load current IS is drawn from supply VS through shunt resistor RS. The voltage drop in shunt resistor VS 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 is: IO = gm (VIN+ – VIN– )(1) where gm = 200µA/V. In the circuit of Figure 1, 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 is 200 µA/V. The complete transfer function for the current measurement amplifier in this application is: VOUT = (IS) (RS) (200µA/V) (RL)(2) The maximum differential input voltage for accurate mea- surements is 0.5V, which produces a 100 µA output current. A differential input voltage of up to 2V will 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, will be zero, but it will not cause damage. BASIC CONNECTION Figure 1 shows the basic connection of the INA138. 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. However, applications with noisy or high- impedance power supplies may require decoupling capaci- tors to reject power-supply noise. Connect bypass capacitors close to the device pins. POWER SUPPLIES The input circuitry of the INA138 can accurately measure beyond its power-supply voltage, V+. For example, the V+ power supply can be 5V, whereas the load power supply voltage is up to +36V (or +60V with the INA168). 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 accu- racy and maximum permissible voltage loss in the measure- ment 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 applica- tions, best performance is attained with an RS value that provides a full-scale shunt voltage range of 50mV to 100mV. Maximum input voltage for accurate measurements is 500mV. RL is chosen to provide the desired full-scale output voltage. The output impedance of the INA138 OUT terminal is very high which permits using values of RL up to 500kΩ 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. |
Similar Part No. - INA138_11 |
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Similar Description - INA138_11 |
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