Datasheet

DAC2902
11
SBAS167C
www.ti.com
As shown in Figure 3, the transformer 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
L
,
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
L
× I
OUTFS
/2. However, in
this case, the two resistors (R
L
) must be used to enable the
necessary DC-current flow for both outputs.
The OPA680 is configured for a gain of two. Therefore,
operating the DAC2902 with a 20mA full-scale output will
produce a voltage output of ±1V. This requires the amplifier
to operate from a dual power supply (±5V). The tolerance of
the resistors typically sets the limit for the achievable com-
mon-mode rejection. An improvement can be obtained by
fine tuning resistor R
4
.
This configuration typically delivers a lower level of ac
performance than the previously discussed transformer solu-
tion because the amplifier introduces another source of
distortion. Suitable amplifiers should be selected based on
their slew-rate, harmonic distortion, and output swing capa-
bilities. 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 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 amplifier can easily be modified to add a
level shift for applications requiring the single-ended output
voltage to be unipolar (that is, swing between 0V and +2V.)
DUAL TRANSIMPEDANCE OUTPUT CONFIGURATION
The circuit example of Figure 5 shows the signal output
currents connected into the summing junctions of the dual
voltage-feedback op amp OPA2680 that is set up as a
transimpedance stage, or I-to-V converter. With this circuit,
the DAC output will be kept at a virtual ground, minimizing
the effects of output impedance variations, which results in
the best DC linearity (INL). As mentioned previously, care
should be taken not to drive the amplifier into slew-rate
limitations, and produce unwanted distortion.
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 DAC2902 generates a differential output
signal of 0.5V
PP
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.
FIGURE 4. Difference Amplifier Provides Differential to
Single-Ended Conversion and DC-Coupling.
FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680
Forms Differential Transimpedance Amplifier.
FIGURE 3. Differential Output Configuration Using an RF
Transformer.
DAC2902
I
OUT
I
OUT
1:1
ADTT1-1
(Mini-Circuits)
R
L
50Ω
R
L
50Ω
R
S
50Ω
R
DIFF
100Ω
I
OUT
I
OUT
DAC2902
R
L
26.1Ω
R
L
28.7Ω
R
4
402Ω
R
3
200Ω
R
2
402Ω
R
1
200Ω
OPA680
C
OPT
+5V
V
OUT
–5V
1/2
OPA2680
1/2
OPA2680
DAC2902
–V
OUT
= I
OUT
• R
F
1
–V
OUT
= I
OUT
• R
F
2
R
F1
R
F2
C
F1
C
F2
C
D1
C
D2
I
OUT
I
OUT
50Ω
50Ω
–5V
+5V