DAC250Y BURR-BROWN [Burr-Brown Corporation], DAC250Y Datasheet - Page 11

• Manufacturer Part Number: DAC250Y
• Description: Dual, 12-Bit, 125MSPS DIGITAL-TO-ANALOG CONVERTER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

FIGURE 4. Difference Amplifier Provides Differential to
DAC2902
As shown in Figure 3, the transformer’s center tap is con-
nected to ground. This forces the voltage swing on I
I
may be replaced with one, R
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
this case, the two resistors (R
necessary DC-current flow for both outputs.
FIGURE 3. Differential Output Configuration Using an RF
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 0.5Vp-p at the load resistors, R
values shown were selected to result in a symmetric 25
loading for each of the current outputs since the input
impedance of the difference amplifier is in parallel to resis-
tors R
OUT
SBAS167A
to be centered at 0V. In this case the two resistors, R
L
DAC2902
DAC2902
, and should be considered.
I
I
OUT
OUT
Transformer.
Single-Ended Conversion and DC-Coupling.
I
I
OUT
OUT
R
26.1
C
L
OPT
R
100
R
50
DIFF
L
R
28.7
DIFF
L
200
200
L
R
R
) must be used to enable the
1
3
, or omitted altogether. This
R
50
L
L
• I
(Mini-Circuits)
R
402
OUTFS
OPA680
–5V
4
ADTT1-1
402
R
1:1
2
+5V
/2. However, in
L
. The resistor
OUT
V
OUT
R
50
S
and
L
,
FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680
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 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 R
This configuration typically delivers a lower level of ac
performance than the previously discussed transformer solu-
tion because the amplifier introduces another source of dis-
tortion. 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 im-
proved by adding a small capacitor (C
outputs I
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 amplifier can easily be modified to add a level
shift for applications requiring the single-ended output volt-
age to be unipolar, i.e., 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 cir-
cuit, the DAC’s 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 previ-
ously, care should be taken not to drive the amplifier into
slew-rate limitations, and produce unwanted distortion.
OUT
DAC2902
I
I
OUT
OUT
Forms Differential Transimpedance Amplifier.
and I
C
C
OUT
D1
D2
50
50
, as shown in Figure 4). This will
4
.
OPA2680
OPA2680
1/2
1/2
+5V
R
C
R
C
–5V
F1
F1
F2
F2
DIFF
–V
–V
OUT
OUT
) between the
= I
= I
OUT
OUT
• R
• R
F
F
1
2
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