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DAC2902Y Fiches technique(PDF) 10 Page - Burr-Brown (TI) |
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DAC2902Y Fiches technique(HTML) 10 Page - Burr-Brown (TI) |
10 / 17 page DAC2902 10 SBAS167A The value of the load resistance is limited by the output compliance specification of the DAC2902. To maintain specified linearity performance, the voltage for IOUT and IOUT should not exceed the maximum allowable compliance range. The two single-ended output voltages can be combined to find the total differential output swing: VOUTDIFF= VOUT – V OUT = (2 • Code– 4095) 4096 • I OUTFS• RLOAD(7) ANALOG OUTPUTS The DAC2902 provides two complementary current out- puts, IOUT and IOUT. The simplified circuit of the analog output stage representing the differential topology is shown in Figure 2. The output impedance of IOUT and IOUT results from the parallel combination of the differential switches, along with the current sources and associated parasitic capacitances. be adapted to the output of the DAC2902 by selecting a suitable transformer while maintaining optimum voltage levels at IOUT and IOUT. Furthermore, using the differential output configuration in combination with a transformer will be instrumental for achieving excellent distortion perfor- mance. Common-mode errors, such as even-order harmon- ics or noise, can be substantially reduced. This is particularly the case with high output frequencies. For those applications requiring the optimum distortion and noise performance, it is recommended to select a full-scale output of 20mA. A lower full-scale range down to 2mA may be considered for applications that require a low power consumption, but can tolerate a slightly reduced perfor- mance level. OUTPUT CONFIGURATIONS The current outputs of the DAC2902 allow for a variety of configurations, some of which are illustrated in Table I. As mentioned previously, utilizing the converter’s differential outputs will yield the best dynamic performance. Such a differential output circuit may consist of an RF transformer or a differential amplifier configuration. The transformer configuration is ideal for most applications with ac coupling, while op amps will be suitable for a DC-coupled configura- tion. The signal voltage swing that may develop at the two outputs, IOUT and IOUT, is limited by a negative and positive compliance. The negative limit of –1V is given by the breakdown voltage of the CMOS process, and exceeding it will compromise the reliability of the DAC2902, or even cause permanent damage. With the full-scale output set to 20mA, the positive compliance equals 1.25V, operating with an analog supply of +VA = 5V. Note that the compliance range decreases to about 1V for a selected output current of IOUTFS = 2mA. Care should be taken that the configuration of DAC2902 does not exceed the compliance range to avoid degradation of the distortion performance and integral lin- earity. Best distortion performance is typically achieved with the maximum full-scale output signal limited to approximately 0.5Vp-p. This is the case for a 50 Ω doubly-terminated load and a 20mA full-scale output current. A variety of loads can The single-ended configuration may be considered for appli- cations requiring a unipolar output voltage. Connecting a resistor from either one of the outputs to ground will convert the output current into a ground-referenced voltage signal. To improve on the DC linearity by maintaining a virtual ground, an I-to-V or op-amp configuration may be consid- ered. DIFFERENTIAL WITH TRANSFORMER Using an RF transformer provides a convenient way of convert- ing the differential output signal into a single-ended signal while achieving excellent dynamic performance (see Figure 3). The appropriate transformer should be carefully selected based on the output frequency spectrum and impedance requirements. The differential transformer configuration has the benefit of significantly reducing common-mode signals, thus improving the dynamic performance over a wide range of frequencies. Furthermore, by selecting a suitable impedance ratio (winding ratio), the transformer can be used to provide optimum imped- ance matching while controlling the compliance voltage for the converter outputs. The model shown, ADTT1-1 (by Mini- Circuits), has a 1:1 ratio and may be used to interface the DAC2902 to a 50 Ω load. This results in a 25Ω load for each of the outputs, IOUT and IOUT. The output signals are ac coupled and inherently isolated because of its magnetic coupling. FIGURE 2. Equivalent Analog Output. I OUT I OUT DAC2902 R L R L +V A INPUT CODE (D11 - D0) IOUT IOUT 1111 1111 1111 20mA 0mA 1000 0000 0000 10mA 10mA 0000 0000 0000 0mA 20mA TABLE I. Input Coding Versus Analog Output Current. |
Numéro de pièce similaire - DAC2902Y |
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Description similaire - DAC2902Y |
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