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AD9708ARRL Arkusz danych(PDF) 11 Page - Analog Devices |
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AD9708ARRL Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 16 page AD9708 –11– REV. B should be considered. The necessity and value of this resistor will be dependent upon the logic family used. For a more detailed discussion of the implementation and construction of high speed, mixed signal printed circuit boards, refer to Analog Devices’ application notes AN-280 and AN-333. DIFFERENTIAL OUTPUT CONFIGURATIONS For applications requiring the optimum dynamic performance and/or a bipolar output swing, a differential output configura- tion is suggested. A differential output configuration may con- sists of either an RF transformer or a differential op amp configuration. The transformer configuration is well suited for ac coupling applications. It provides the optimum high fre- quency performance due to its excellent rejection of common- mode distortion (i.e., even-order harmonics) and noise over a wide frequency range. It also provides electrical isolation and the ability to deliver twice the power to the load (i.e., assuming no source termination). The differential op amp configuration is suitable for applications requiring dc coupling, a bipolar output, signal gain, and/or level shifting. Figure 23 shows the AD9708 in a typical transformer coupled output configuration. The center-tap on the primary side of the transformer must be connected to ACOM to provide the necessary dc current path for both IOUTA and IOUTB. The complemen- tary voltages appearing at IOUTA and IOUTB (i.e., VOUTA and VOUTB) swing symmetrically around ACOM and should be maintained within the specified output compliance range of the AD9708. A differential resistor, RDIFF, may be inserted in applications in which the output of the transformer is connected to the load, RLOAD, via a passive reconstruction filter or cable. RDIFF is determined by the transformer’s impedance ratio and provides the proper source termination. Note that approxi- mately half the signal power will be dissipated across RDIFF. RLOAD AD9708 22 21 MINI-CIRCUITS T1-1T OPTIONAL RDIFF IOUTA IOUTB Figure 23. Differential Output Using a Transformer An op amp can also be used to perform a differential to single- ended conversion as shown in Figure 24. The AD9708 is configured with two equal load resistors, RLOAD, of 25 Ω. The differential voltage developed across IOUTA and IOUTB is converted to a single-ended signal via the differential op amp configuration. An optional capacitor can be installed across IOUTA and IOUTB forming a real pole in a low-pass filter. The addition of this capacitor also enhances the op amps distortion performance by preventing the DACs high slewing output from overloading the op amp’s input. AD9708 22 IOUTA IOUTB 21 COPT 500 225 225 500 25 25 AD8072 Figure 24. DC Differential Coupling Using an Op Amp The common-mode rejection of this configuration is typically determined by the resistor matching. In this circuit, the differ- ential op amp circuit is configured to provide some additional signal gain. The op amp must operate off a dual supply since its output is approximately ±1.0 V. A high speed amplifier capable of preserving the differential performance of the AD9708 while meeting other system level objectives (i.e., cost, power) should be selected. The op amps differential gain, its gain setting resis- tor values and full-scale output swing capabilities should all be considered when optimizing this circuit. The differential circuit shown in Figure 25 provides the neces- sary level-shifting required in a single supply system. In this case, AVDD, which is the positive analog supply for both the AD9708 and the op amp, is also used to level-shift the differ- ential output of the AD9762 to midsupply (i.e., AVDD/2). AD9708 22 IOUTA IOUTB 21 COPT 500 225 225 1k 25 25 AD8072 1k AVDD Figure 25. Single-Supply DC Differential Coupled Circuit AD9708 EVALUATION BOARD General Description The AD9708-EB is an evaluation board for the AD9708 8-bit D/A converter. Careful attention to layout and circuit design, combined with a prototyping area, allows the user to easily and effectively evaluate the AD9708 in any application where high resolution, high speed conversion is required. This board allows the user the flexibility to operate the AD9708 in various configurations. Possible output configurations include transformer coupled, resistor terminated, inverting/noninverting and differential amplifier outputs. The digital inputs are designed to be driven directly from various word generators, with the on-board option to add a resistor network for proper load termination. Provisions are also made to operate the AD9708 with either the internal or external reference, or to exercise the power-down feature. Refer to the application note AN-420 “Using the AD9760/ AD9762/AD9764-EB Evaluation Board” for a thorough description and operating instructions for the AD9708 evalua- tion board. |
Podobny numer części - AD9708ARRL |
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Podobny opis - AD9708ARRL |
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