Datasheet

DAC908
12
SBAS109B
FIGURE 3. Differential Output Configuration Using an RF
Transformer.
FIGURE 4. Difference Amplifier Provides Differential to
Single-Ended Conversion and DC-Coupling.
FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680
Forms Differential Transimpedance Amplifier.
As shown in Figure 3, the transformer’s center tap must be
connected to ground to enable the necessary DC-current
flow for both outputs. Some applications may require a solid
termination, in which case a differential resistor, R
DIFF
, may
be inserted as shown. Note that this will reduce the available
signal power by approximately one half.
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 DAC908 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 resis-
tors R
L
, and should be considered.
The OPA680 is configured for a gain of two. Therefore,
operating the DAC908 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
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 intro-
duce a real pole to create a low-pass filter in order to slew-
limit the DACs fast output signal steps, which 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, 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 junction of the
OPA2680, which is set up as a transimpedance stage, or
I-to-V converter. With this circuit, the DAC’s output will be
kept at a virtual ground, minimizing the effects of output
impedance variations, and resulting in the best DC linearity
(INL). However, as mentioned previously, the amplifier
may be driven into slew-rate limitations, and produce un-
wanted distortion. This may occur especially at high DAC
update rates.
I
OUT
I
OUT
DAC908
1:1
ADT1-1WT
(Mini-Circuits)
50Ω
50Ω
R
L
Optional
R
DIFF
I
OUT
I
OUT
DAC908
R
L
26.1Ω
R
L
28.7Ω
R
4
402Ω
R
3
200Ω
R
2
402Ω
R
1
200Ω
OPA680
C
DIFF
+5V
V
OUT
–5V
1/2
OPA2680
1/2
OPA2680
DAC908
–V
OUT
= I
OUT
• R
F
–V
OUT
= I
OUT
• R
F
R
F1
R
F2
C
F1
C
F2
C
D1
C
D2
I
OUT
I
OUT
50Ω
50Ω
–5V
+5V