SBOS746A June 2016 – February 2021 THS6302
The G.Fast signal input to the THS6302 comes from a high-speed DAC in the DSL ASIC whose interleaving spurs are filtered out using either a 3rd- or 5th-order filter. Digital pre-emphasis can be employed in the DAC output such that the differential line driver compensates for the transmission line cable losses at long distance and high frequency. The THS6302 is operated on a 12-V single supply. Resulting from the single-supply operation, the device input is AC-coupled using a capacitor that blocks any DC current flowing out of the inputs to the adjacent circuitry. The AC-coupling capacitor forms a high-pass filter with the device input impedance. This pole must be set at a frequency low enough to not interfere with the desired xDSL or G.Fast signal.
The THS6302 differential outputs usually drive a 1:n output transformer with a transformer turns ratio that can be changed depending upon the application. The output transformer selected must have low insertion loss in the desired frequency band in order to maintain good multi-tone power rejection (MTPR) for a given line power. The load is expected to be a transmission line with 100-Ω characteristic impedance on the primary side (line load side) of the transformer. Referred to the transformer secondary, the load seen by the amplifier is 1/n2 with 1:n being the transformer turn ratio. Practical limitations force the transformer-turn ratio to be between 1:1 and 1:1.6. At the lighter load seen by the amplifier (1:1), the voltage swing is limited by the class AB output stage and the maximum achievable swing of the amplifier. At the heaviest load (1:1.6), the voltage swing is limited by the current drive capability of the amplifier.
For surge protection, consider adding a gas discharge tube (GDT) on the primary side of the output transformer. The gas discharge tube is required to shunt the large current that could flow through the cables during lightning surge, and protect the device outputs. The secondary protection is also normally added after the series resistance on the secondary transformer side. The secondary protection could be in the form of back to back switching diodes, which also help limit the residual surge current flowing into the device outputs.
For the power-supply bypass, consider using X7R or X5R because of the better stability of these materials over temperature.