DAC904 Datasheet, PDF (12/23 Pages) Burr-Brown (TI) – 14-Bit, 165MSPS DIGITAL-TO-ANALOG CONVERTER

transformer configuration is ideal for most applications with ac
coupling, while op amps will be suitable for a DC-coupled
configuration.
The single-ended configuration (see Figure 6) may be consid-
ered for applications requiring a unipolar output voltage. Con-
necting a resistor from either one of the outputs to ground will
convert the output current into a ground-referenced voltage
signal. To improve on the DC linearity, an I-to-V converter can
be used instead. This will result in a negative signal excursion
and, therefore, requires a dual supply amplifier.
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, as
shown in Figure 3. 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 com-
mon-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.
The model shown in Figure 3 has a 1:1 ratio and may be
used to interface the DAC904 to a 50Ω load. This results in
a 25Ω load for each of the outputs, I_OUT and I_OUT. The output
signals are ac coupled and inherently isolated because of the
transformer's magnetic coupling.
As shown in Figure 3, the transformer’s center tap is con-
nected to ground. This forces the voltage swing on I_OUT and
I_OUT to be centered at 0V. In this case the two resistors, R_S,
may be replaced with one, R_DIFF, or omitted altogether. This
approach should only be used if all components are close to
each other, and if the VSWR is not important. A complete
power transfer from the DAC output to the load can be
realized, but the output compliance range should be ob-
served. Alternatively, if the center tap is not connected, the
signal swing will be centered at R_S • I_OUTFS/2. However, in
this case, the two resistors (R_S) must be used to enable the
necessary DC-current flow for both outputs.
ADT1-1WT
(Mini-Circuits)
1:1
I_OUT
R_S
DAC904
Optional
R_DIFF
50Ω
R_L
I_OUT
R_S
50Ω
FIGURE 3. Differential Output Configuration Using an RF
Transformer.
DIFFERENTIAL CONFIGURATION USING AN OP AMP
If the application requires a DC-coupled output, a difference
amplifier may be considered, as shown in Figure 4. Four
external resistors are needed to configure the voltage-feed-
back op amp OPA680 as a difference amplifier performing
the differential to single-ended conversion. Under the shown
configuration, the DAC904 generates a differential output
signal of 0.5Vp-p at the load resistors, R_L. The resistor values
shown were selected to result in a symmetric 25Ω loading for
each of the current outputs since the input impedance of the
difference amplifier is in parallel to resistors R_L, and should
be considered.
I_OUT
DAC904
I_OUT
R₁
200Ω
C_DIFF
R₃
200Ω
R_L
26.1Ω
R_L
28.7Ω
R₂
402Ω
OPA680
–5V +5V
R₄
402Ω
V_OUT
FIGURE 4. Difference Amplifier Provides Differential to Single-
Ended Conversion and AC-Coupling.
The OPA680 is configured for a gain of 2. Therefore, oper-
ating the DAC904 with a 20mA full-scale output will produce
a voltage output of ±1V. This requires the amplifier to operate
off of a dual power supply (±5V). The tolerance of the
resistors typically sets the limit for the achievable common-
mode rejection. An improvement can be obtained by fine
tuning resistor R₄.
This configuration typically delivers a lower level of ac perfor-
mance than the previously discussed transformer solution
because the amplifier introduces another source of distor-
tion. Suitable amplifiers should be selected based on their
slew-rate, harmonic distortion, and output swing capabilities.
High-speed amplifiers like the OPA680 or OPA687 may be
considered. The ac performance of this circuit may be
improved by adding a small capacitor, C_DIFF, between the
outputs I_OUT and I_OUT, as shown in Figure 4. This will introduce
a real pole to create a low-pass filter in order to slew-limit the
DAC’s fast output signal steps that otherwise could drive the
amplifier into slew-limitations or into an overload condition;
both would cause excessive distortion. The difference ampli-
fier can easily be modified to add a level shift for applications
requiring the single-ended output voltage to be unipolar, i.e.,
swing between 0V and +2V.
12
DAC904
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