Datasheet

Differential With Transformer
IOUT1
1:1
IOUT2
50 Ω
50 Ω
R
LOAD
50 Ω
100 Ω
AGND
IOUT1
4:1
IOUT2
100 Ω
100 Ω
R
LOAD
50 Ω
AGND
DAC5672A
SLAS528A – SEPTEMBER 2007 – REVISED MAY 2009 ................................................................................................................................................... www.ti.com
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
op-amp 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 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 22 and Figure 23 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 has a better output
distortion, but the 4:1 transformer has 6 dB higher output power.
Figure 22. Driving a Doubly-Terminated 50- Ω Cable Using a 1:1 Impedance Ratio Transformer
Figure 23. Driving a Doubly-Terminated 50- Ω Cable Using a 4:1 Impedance Ratio Transformer
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