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AD768AR Folha de dados(PDF) 6 Page - Analog Devices |
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AD768AR Folha de dados(HTML) 6 Page - Analog Devices |
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6 / 20 page  REV. B –6– AD768 IREFIN – mA 550 500 300 1.0 7.0 2.0 3.0 4.0 5.0 6.0 450 400 350 Figure 3. Power Dissipation vs. IREFIN Current Note the AD768 is optimized for operation at an input current of 5 mA. Both linearity and dynamic performance at other input currents may be somewhat degraded. Figure 4 shows typical dc linearity over a range of input currents. Figure 5 shows typical SFDR (to Nyquist) performance over a range of input currents and CLOCK input rates for a 1 MHz output frequency. IREFIN – mA 10 0 1.0 7.0 2.0 3.0 4.0 5.0 6.0 9 6 3 2 1 8 7 5 4 INL DNL Figure 4. INL/DNL vs. IREFIN Current IREFIN – mA –85 –55 1.0 7.0 2.0 3.0 4.0 5.0 6.0 –80 –75 –70 –65 –60 CLOCK = 10 MSPS CLOCK = 20 MSPS CLOCK = 30 MSPS CLOCK = 40 MSPS Figure 5. SFDR (to Nyquist) vs. IREFIN @ FOUT = 1 MHz DAC TRANSFER FUNCTION The AD768 may be used in either current-output mode with the output connected to a virtual ground, or voltage-output mode with the output connected to a resistive load. In current output mode, IOUT = (DAC CODE/65536) × (I REFIN × 4) In voltage output mode, VOUT = IOUT × R LOAD RLAD where: DAC CODE is the decimal representation of the DAC inputs; an integer between 0 and 65535. IREFIN is the current applied at the IREFIN pin, determined by VREF/RREF. Substituting for IOUT and IREFIN, VOUT = –VREF × (DAC CODE/65536) × 4 × [(R LOAD RLAD)/RREF] These equations clarify an important aspect of the AD768 transfer function; the full-scale current output of the DAC is proportional to a current input. The voltage output is then a function of the ratio of (RLOAD RLAD)/RREF, allowing for cancel- lation of resistor drift by selection of resistors with matched characteristics. REFERENCE INPUT The IREFIN pin is a current input node with low impedance to REFCOM. This input current sets the magnitude of the DAC current sources such that the full-scale output current is exactly four times the current applied at IREFIN. For the nominal in- put current of 5 mA, the nominal full-scale output current is 20 mA. The 5 mA reference input current can be generated from the on-chip 2.5 V reference with an external resistor of 500 Ω from REFOUT to IREFIN. If desired, a variety of external reference voltages may be used based on the selection of an appropriate resistor. However, to maintain stability of the reference ampli- fier, the external impedance at IREFIN must be kept below 1k Ω. 5 6 REFCOM IREFIN VEE VEE IFB 5mA Figure 2. Equivalent Reference Input Circuit The IREFIN current can be varied from 1 mA to 7 mA which subsequently will result in a proportional change in the DAC full-scale. Since the operating currents within the DAC vary with IREFIN, so does the power dissipation. Figure 3 illustrates that relationship. |
Nº de peça semelhante - AD768AR |
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Descrição semelhante - AD768AR |
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