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DAC2900-EVM 数据表(PDF) 11 Page - Burr-Brown (TI) |
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DAC2900-EVM 数据表(HTML) 11 Page - Burr-Brown (TI) |
11 / 17 page DAC2900 11 SBAS166B 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, RL, 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 RL • IOUTFS/2. However, in this case, the two resistors, RL, 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 DAC2900 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 outputs IOUT and IOUT (as shown in Figure 4). This will introduce a real pole to create a low-pass filter in order to slew-limit the DAC’s fast output signal steps, which other- wise could drive the amplifier into slew-limitations or into an overload condition; both would cause excessive distor- tion. 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 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. 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 DAC2900 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 25 Ω 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. FIGURE 3. Differential Output Configuration Using an RF Transformer. FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680 Forms Differential Transimpedance Amplifier. FIGURE 4. Difference Amplifier Provides Differential to Single-Ended Conversion and DC-Coupling. DAC2900 I OUT I OUT 1:1 ADTT1-1 (Mini-Circuits) R L 50 Ω R L 50 Ω R S Optional R DIFF I OUT I OUT DAC2900 R L 26.1 Ω R L 28.7 Ω R 4 402 Ω R 3 200 Ω R 2 402 Ω R 1 200 Ω OPA680 C OPT +5V V OUT –5V 1/2 OPA2680 1/2 OPA2680 DAC2900 –V OUT = IOUT • RF1 –V OUT = IOUT • RF2 R F1 R F2 C F1 C F2 C D1 C D2 I OUT I OUT 50 Ω 50 Ω –5V +5V |
类似零件编号 - DAC2900-EVM |
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类似说明 - DAC2900-EVM |
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