DAC900-Q1 Datasheet, PDF (16/26 Pages) Texas Instruments – 10-BIT 165-MSPS DIGITAL-TO-ANALOG CONVERTER

DAC900-Q1
SBAS505 – JUNE 2010
ADT1-1WT
(Mini-Circuits)
1:1
I_OUT
R_S
DAC900
Optional
R_DIFF
50Ω
R_L
I_OUT
R_S
50Ω
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Figure 3. Differential Output Configuration Using an RF Transformer
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-feedback 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, 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.
R₂
402Ω
I_OUT
DAC900
I_OUT
R₁
200Ω
C_DIFF
R₃
200Ω
OPA680
–5V +5V
V_OUT
R_L
26.1Ω
R_L
28.7Ω
R₄
402Ω
Figure 4. Difference Amplifier Provides Differential to Single-Ended Conversion and DC-Coupling
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
solution because the amplifier introduces another source of distortion. 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, 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 slewlimit 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 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.
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