8.1.1 Operation

Figure 9 illustrates the basic circuit diagram for both the INA138-Q1 and INA168-Q1. Load current I_S is drawn from supply V_P through shunt resistor R_S. The voltage drop in the shunt resistor 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, I_O, to a voltage, V_OUT, at the OUT pin. The transfer function for the INA1x8-Q1 is shown in Equation 3:

In the circuit of Figure 9, the input voltage, (V_IN+ − V_IN−), is equal to I_S × R_S. The output voltage, V_OUT, is equal to I_O × R_L. The transconductance, g_m, of the INA1x8-Q1 is 200 μA/V. The complete transfer function for the current measurement amplifier in this application is shown in Equation 4:

The maximum differential input voltage for accurate measurements is 0.5 V, producing a 100-μA output current. A differential input voltage of up to 2 V does not cause damage. Differential measurements (V_IN+ and V_IN− pins) must be unipolar, with a more-positive voltage applied to the V_IN+ pin. If a more-negative voltage is applied to the V_IN+ pin, I_O goes to zero, but no damage occurs.

1. Maximum V_P and V+ voltage is 60 V with INA168-Q1.

Figure 9. Basic Circuit Connections