The transfer function of the circuit in Figure 8-1 is given in Equation 1.

The load current (I_LOAD) produces a voltage drop across the shunt resistor (R_SHUNT). The load current is set from 0A to 1A. To keep the shunt voltage below 100 mV at maximum load current, the largest shunt resistor is defined using Equation 2.

Using Equation 2, R_SHUNT is calculated to be 100mΩ. The voltage drop produced by I_LOAD and R_SHUNT is amplified by the OPA323 to produce an output voltage of approximately 0V to 4.9V. The gain needed by the OPA323 to produce the necessary output voltage is calculated using Equation 3.

Using Equation 3, the required gain is calculated to be 49V/V, which is set with resistors R_F and R_G. Equation 4 sizes the resistors R_F and R_G, to set the gain of the OPA323 to 49V/V.

Selecting R_F as 57.6kΩ and R_G as 1.2kΩ provides a combination that equals 49V/V. Figure 8-2 shows the measured transfer function of the circuit shown in Figure 8-1. Notice that the gain is only a function of the feedback and gain resistors. This gain is adjusted by varying the ratio of the resistors and the actual resistors values are determined by the impedance levels that the designer wants to establish. The impedance level determines the current drain, the effect that stray capacitance has, and a few other behaviors. There is no optimal impedance selection that works for every system; choose an impedance that is best for the system parameters.