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Output Configurations

Differential With Transformer

1:1

50

Ω

50

Ω

RLOAD

50

Ω

100

Ω

AGND

IOUT2

IOUT1

DAC5662-EP

SGLS340A – JUNE 2006 – REVISED OCTOBER 2006

APPLICATION INFORMATION (continued)

The current outputs of the DAC5662 allow for a variety of configurations. As mentioned previously, utilizing the

converter’s differential outputs yield the best dynamic performance. Such a differential output circuit may consist

of an RF transformer or a differential amplifier configuration. The transformer configuration is ideal for most

applications with ac coupling, while operational amplifiers are suitable for a dc-coupled configuration.

The single-ended configuration may be considered for applications requiring a unipolar output voltage.

Connecting a resistor from either one of the outputs to ground converts the output current into a

ground-referenced voltage signal. To improve on the dc linearity by maintaining a virtual ground, an I-to-V or

operational amplifier configuration may be considered.

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 should 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 be used to provide optimum impedance matching while

controlling the compliance voltage for the converter outputs.

Figure 21 and Figure 22 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 leads to a 0.5-VPP output for a 1:1 transformer and

a 1-VPP output for a 4:1 transformer. In general, the 1:1 transformer configuration has slightly better output

distortion, but the 4:1 transformer has 6-dB higher output power.

Figure 21. Driving a Doubly-Terminated 50-

Ω Cable Using a 1:1 Impedance Ratio Transformer

20

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