Datasheet

DAC2904
I
OUT
R
L
50W
R
L
50W
R
S
50W
R
DIFF
100W
I
OUT
ADTT1-1
(Mini-Circuits)
1:1
DAC2904
www.ti.com .............................................................................................................................................. SBAS198C –AUGUST 2001–REVISED OCTOBER 2009
OUTPUT CONFIGURATIONS DIFFERENTIAL WITH TRANSFORMER
The current outputs of the DAC2904 allow for a Using an RF transformer provides a convenient way
variety of configurations, some of which are illustrated of converting the differential output signal into a
in Table 1. As mentioned previously, utilizing the single-ended signal while achieving excellent
converter differential outputs will yield the best dynamic performance (see Figure 20). The
dynamic performance. Such a differential output appropriate transformer should be carefully selected
circuit may consist of an RF transformer or a based on the output frequency spectrum and
differential amplifier configuration. The transformer impedance requirements. The differential transformer
configuration is ideal for most applications with ac configuration has the benefit of significantly reducing
coupling, while op amps will be suitable for a common-mode signals, thus improving the dynamic
dc-coupled configuration. performance over a wide range of frequencies.
Furthermore, by selecting a suitable impedance ratio
Table 1. Input Coding vs Analog Output Current (winding ratio), the transformer can be used to
provide optimum impedance matching while
INPUT CODE (D13 - D0) I
OUT
I
OUT
controlling the compliance voltage for the converter
11 1111 1111 1111 20mA 0mA
outputs. The model shown, ADTT1-1 (by
10 0000 0000 0000 10mA 10mA
Mini-Circuits), has a 1:1 ratio and may be used to
00 0000 0000 0000 0mA 20mA
interface the DAC2904 to a 50Ω load. This results in
a 25Ω load for each of the outputs, I
OUT
and I
OUT
.
The single-ended configuration may be considered
The output signals are ac-coupled and inherently
for applications requiring a unipolar output voltage.
isolated because of its magnetic coupling.
Connecting a resistor from either one of the outputs
As shown in Figure 20, the transformer center tap is
to ground will convert the output current into a
connected to ground. This forces the voltage swing
ground-referenced voltage signal. To improve on the
on I
OUT
and I
OUT
to be centered at 0V. In this case
dc linearity by maintaining a virtual ground, an I-to-V
the two resistors, R
L
, may be replaced with one,
or op amp configuration may be considered.
R
DIFF
, or omitted altogether. This approach should
space
only be used if all components are close to each
other, and if the VSWR is not important. A complete
space
power transfer from the DAC output to the load can
space
be realized, but the output compliance range should
be observed. Alternatively, if the center tap is not
space
connected, the signal swing will be centered at (R
L
×
I
OUTFS
/2). However, in this case, the two load
space
resistors, R
L
, must be used to enable the necessary
dc-current flow for both outputs.
Figure 20. Differential Output Configuration Using an RF Transformer
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