8.1.1 Operation

Figure 9 illustrates the basic circuit diagram for both the INA138 and INA168 devices. Load current I_S is drawn from supply V_S through shunt resistor R_S. The voltage drop in shunt resistor V_S is forced across R_G1 by the internal op amp, causing current to flow into the collector of Q1. External resistor R_L converts the output current to a voltage, V_OUT, at the OUT pin. The transfer function for the INA138 device is:

where g_m = 200 µA/V.

In the circuit of Figure 9, the input voltage, (V_IN+ – V_IN–), is equal to I_S × R_S and the output voltage, V_OUT, is equal to I_O × R_L. The transconductance, g_m, of the INA138 device is 200 µA/V. The complete transfer function for the current measurement amplifier in this application is:

The maximum differential input voltage for accurate measurements is 0.5 V, which produces a 100-µA output current. A differential input voltage of up to 2 V 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, I_O, will be zero, but it will not cause damage.

1. Maximum V_P and V+ voltage is 60 V with INA168.

Figure 9. Basic Circuit Connections