The closed-loop gain equation for a differential line driver such as the THS6222 is given as A_V = 1 + 2 × (R_F / R_G), where R_F = R_F1 = R_F2. The THS6222 is a current-feedback amplifier and thus the bandwidth of the closed-loop configuration is set by the value of the R_F resistor. This advantage of the current-feedback architecture allows for flexibility in setting the differential gain by choosing the value of the R_G resistor without reducing the bandwidth as is the case with voltage-feedback amplifiers. The THS6222 is designed to provide optimal bandwidth performance with R_F1 = R_F2 = 1.24 kΩ. To configure the device in a gain of 10 V/V, the R_G resistor is chosen to be 274 Ω. See the TI Precision Labs for more details on how to choose the R_F resistor to optimize the performance of a current-feedback amplifier.

Often, a key requirement for PLC applications is the out-of-band suppression specifications. The in-band frequencies carry the encoded data with a certain power level. The line driver must not generate any spurs beyond a certain power level outside the in-band spectrum. In the design requirements of this application example, the minimum out-of-band suppression specification of 35 dB means there must be no frequency spurs in the out-of-band spectrum beyond the –80 dBm/Hz power spectral density, considering the in-band power spectral density is –50 dBm/Hz.

The circuit shown in Figure 8-2 measures the out-of-band suppression specification. The minor difference in components between the circuits of Figure 8-1 and Figure 8-2 does not have any significant impact on the out-of-band suppression results.

Figure 8-2 Measurement Test Circuit for Out-of-Band Suppression