Selecting RF as 255 kΩ and RG as 7.5 kΩ provides a combination that equals 35 V/V. Figure 32 shows the measured transfer function of the circuit shown in Figure 31. 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, you must choose an impedance that is ideal for your system parameters.