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DAC904EG4 bảng dữ liệu(PDF) 11 Page - Texas Instruments |
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DAC904EG4 bảng dữ liệu(HTML) 11 Page - Texas Instruments |
11 / 23 page DAC904 11 SBAS095C www.ti.com DAC TRANSFER FUNCTION The total output current, IOUTFS, of the DAC904 is the sum- mation of the two complementary output currents: IOUTFS = IOUT + IOUT (1) The individual output currents depend on the DAC code and can be expressed as: IOUT = IOUTFS • (Code/16384) (2) IOUT = IOUTFS • (16383 – Code/16384) (3) where ‘Code’ is the decimal representation of the DAC data input word. Additionally, IOUTFS is a function of the reference current IREF, which is determined by the reference voltage and the external setting resistor, RSET. IOUTFS = 32 • IREF = 32 • VREF/RSET (4) In most cases the complementary outputs will drive resistive loads or a terminated transformer. A signal voltage will develop at each output according to: VOUT = IOUT • RLOAD (5) VOUT = IOUT • RLOAD (6) The value of the load resistance is limited by the output compliance specification of the DAC904. To maintain speci- fied 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: (7) VV V Code IR OUTDIFF OUT OUT OUTFS LOAD == • •• – ( – ) 2 16383 16384 ANALOG OUTPUTS The DAC904 provides two complementary current outputs, IOUT and IOUT. The simplified circuit of the analog output stage representing the differential topology is shown in Figure 2. The output impedance of 200k Ω 12pF for I OUT and IOUT results from the parallel combination of the differen- tial switches, along with the current sources and associated parasitic capacitances. 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 break- down voltage of the CMOS process, and exceeding it will compromise the reliability of the DAC904, or even cause permanent damage. With the full-scale output set to 20mA, the positive compliance equals 1.25V, operating with FIGURE 2. Equivalent Analog Output. I OUT I OUT DAC904 R L R L +V A +VD = 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 the DAC904 does not exceed the compliance range to avoid degradation of the distortion performance and integral linearity. Best distortion performance is typically achieved with the maximum full-scale output signal limited to approximately 0.5V. This is the case for a 50 Ω doubly-terminated load and a 20mA full-scale output current. A variety of loads can be adapted to the output of the DAC904 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 instru- mental for achieving excellent distortion performance. Com- mon-mode errors, such as even-order harmonics or noise, can be substantially reduced. This is particularly the case with high output frequencies and/or output amplitudes below full-scale. 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 reduced performance level. INPUT CODE (D13 - D0) IOUT IOUT 11 1111 1111 1111 20mA 0mA 10 0000 0000 0000 10mA 10mA 00 0000 0000 0000 0mA 20mA TABLE I. Input Coding versus Analog Output Current. OUTPUT CONFIGURATIONS The current output of the DAC904 allows for a variety of configurations, some of which are illustrated below. As men- tioned previously, utilizing the converter’s differential outputs will yield the best dynamic performance. Such a differential output circuit may consist of an RF transformer (see Figure 3) or a differential amplifier configuration (see Figure 4). The |
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