As shown in Figure 3, the transformer’s center tap is con-
nected to ground. This forces the voltage swing on IOUT and
IOUT 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.
The OPA680 is configured for a gain of two. Therefore,
operating the DAC900 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
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, CDIFF, between the
ADT1-1WT
outputs IOUT and IOUT, as shown in Figure 4. This will intro-
(Mini-Circuits)
1:1
duce a real pole to create a low-pass filter in order to slew-
limit the DACs fast output signal steps that otherwise could
drive the amplifier into slew-limitations or into an overload
condition; both would cause excessive distortion. The differ-
ence 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.
IOUT
RS
50Ω
Optional
RDIFF
DAC900
RL
IOUT
RS
50Ω
DUAL TRANSIMPEDANCE OUTPUT CONFIGURATION
FIGURE 3. Differential Output Configuration Using an RF
Transformer.
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.
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 DAC900 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 resis-
tors RL, and should be considered.
+5V
50Ω
1/2
OPA2680
–VOUT = IOUT • RF
RF1
CF1
DAC900
R2
402Ω
IOUT
CD1
R1
200Ω
IOUT
RF2
CF2
VOUT
DAC900
IOUT
OPA680
R3
200Ω
CDIFF
CD2
IOUT
–5V +5V
RL
28.7Ω
R4
402Ω
RL
26.1Ω
1/2
OPA2680
–VOUT = IOUT • RF
50Ω
–5V
FIGURE 4. Difference Amplifier Provides Differential to
Single-Ended Conversion and DC-Coupling.
FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680
Forms Differential Transimpedance Amplifier.
DAC900
SBAS093B
13
TI [ TEXAS INSTRUMENTS ]
