DIFFERENTIAL CONFIGURATION USING AN OP AMP
transformer configuration is ideal for most applications with ac
coupling, while op amps will be suitable for a DC-coupled
configuration.
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, RL. 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 RL, and should
be considered.
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, IOUT and IOUT. The output
signals are ac coupled and inherently isolated because of the
transformer's magnetic coupling.
R2
402Ω
R1
200Ω
IOUT
VOUT
DAC904
IOUT
OPA680
R3
200Ω
CDIFF
–5V +5V
RL
28.7Ω
R4
402Ω
RL
26.1Ω
FIGURE 4. Difference Amplifier Provides Differential to Single-
Ended Conversion and AC-Coupling.
As shown in Figure 3, the transformer’s center tap is con-
nected to ground. This forces the voltage swing on IOUT and
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.
I
OUT to be centered at 0V. In this case the two resistors, RS,
may be replaced with one, RDIFF, 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 RS • IOUTFS/2. However, in
this case, the two resistors (RS) must be used to enable the
necessary DC-current flow for both outputs.
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, CDIFF, between the
ADT1-1WT
(Mini-Circuits)
1:1
IOUT
outputs IOUT and IOUT, as shown in Figure 4. This will introduce
RS
50Ω
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.
Optional
RDIFF
DAC904
RL
IOUT
RS
50Ω
FIGURE 3. Differential Output Configuration Using an RF
Transformer.
DAC904
12
SBAS095C
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