7.4.4 Differential With Transformer

Using an RF transformer provides a convenient way of converting the differential output signal into a single-ended signal while achieving excellent dynamic performance. The appropriate transformer must be carefully selected based on the output frequency spectrum and impedance requirements.

The differential transformer configuration has the benefit of significantly reducing common-mode signals, thus improving the dynamic performance over a wide range of frequencies. Furthermore, by selecting a suitable impedance ratio (winding ratio) the transformer can provide optimum impedance matching while controlling the compliance voltage for the converter outputs.

Figure 20 and Figure 21 show 50-Ω doubly-terminated transformer configurations with 1:1 and 4:1 impedance ratios, respectively. Note that the center tap of the primary input of the transformer has to be grounded to enable a dc-current flow. Applying a 20-mA full-scale output current would lead to a 0.5-V_PP output for a 1:1 transformer and a 1-V_PP output for a 4:1 transformer. In general, the 1:1 transformer configuration will have slightly better output distortion, but the 4:1 transformer will have 6 dB higher output power.

Figure 20. Driving a Doubly-Terminated 50-Ω Cable Using a 1:1 Impedance Ratio Transformer

Figure 21. Driving a Doubly-Terminated 50-Ω Cable Using a 4:1 Impedance Ratio Transformer