The closed-loop gain equation for a differential line driver such as the THS6232 is given as:

where R_F = R_F1 = R_F2.

The THS6232 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 THS6232 is designed to provide excellent bandwidth performance with R_F1 = R_F2 = 1.24kΩ. To configure the device in a gain of 10V/V, use an R_G resistor value of 274Ω. For operation in ultra-low-bias mode, use a minimum R_F1 = R_F2 = 2kΩ, and for device gain of 10V/V, use R_G = 442Ω. See Current feedback amplifiers - Overview and compensation techniques video 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 35dB means there must be no frequency spurs in the out-of-band spectrum beyond the –45dBm transmit power, considering the in-band transmit power is –10dBm.

The circuit shown in Figure 7-2 measures the out-of-band suppression specification. The minor difference in components between the circuits of Figure 7-1 and Figure 7-2 does not have any significant impact on the out-of-band suppression results.

Figure 7-2 Measurement Test Circuit for Out-of-Band Suppression