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AD766AN Datasheet(PDF) 5 Page - Analog Devices |
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AD766AN Datasheet(HTML) 5 Page - Analog Devices |
5 / 8 page –5– REV. A The digital ground pin returns ground current from the digital logic portions of the AD766 circuitry. This pin should be con- nected to the digital common point in the system. As illustrated in Figure 5, the analog and digital grounds should be connected together at one point in the system. Figure 5. Recommended Circuit Schematic POWER SUPPLIES AND DECOUPLING The AD766 has four power supply input pins. ±V S provide the supply voltages to operate the linear portions of the DAC in- cluding the voltage reference, output amplifier and control am- plifier. The ±V S supplies are designed to operate from ± 5 V to ±12 V. The ±V L supplies operate the digital portions of the chip, in- cluding the input shift register and the input latching circuitry. The ±V L supplies are also designed to operate from ± 5 V to ±12 V. To assure freedom from latch-up, –V L should never go more negative than –VS. Special restrictions on power supplies apply to extended tem- perature range versions of the AD766 that do not apply to the commercial AD766J. First, supplies must be symmetric. That is, +VS = –VS and +VL = –VL . Each supply must independently meet this equality within ±5%. Since we require that –V S ≤ –VL to guarantee latch-up immunity, this symmetry principle implies that the positive analog supply must be greater than or equal to the positive digital supply, i.e., VS ≥ –VL for extended-temper- ature range parts. In other words, the digital supply range must be inside the analog supply range. Second, the internal op amp’s performance in generating voltage outputs is only guaranteed if +VS ≥ 7 V (and –V S ≤ –7 V, by the symmetry principle). These constraints do not apply to the AD766J. Decoupling capacitors should be used on all power supply pins. Furthermore, good engineering practice suggests that these ca- pacitors be placed as close as possible to the package pins as well as the common points. The logic supplies, ±V L, should be decoupled to digital common; and the analog supplies, ±V S, should be decoupled to analog common. The use of four separate power supplies will reduce feedthrough from the digital portion of the system to the linear portions of the system, thus contributing to the performance as tested. However, four separate voltage supplies are not necessary for good circuit performance. For example, Figure 6 illustrates a For applications requiring broader bandwidths and/or even lower noise than that afforded by the AD766’s internal op amp, an external op amp can easily by used in its place. IOUT (Pin 13) drives the negative (inverting) input terminal of the external op amp, and its external voltage output is connected to the feed- back resistor pin, RF (Pin 10). To insure that the AD766’s un- used internal op amp remains in a closed-loop configuration, VOUT (Pin 9) should be tied to the summing junction pin, SJ (Pin 11). As an example, Figure 3 shows the AD766 using the AD744 op amp as an external current-to-voltage converter. In this invert- ing configuration, the AD744 will provide the same ±3 V out- put as the internal op amp would have. Other recommended amplifiers include the AD845 and AD846. Note that a single pole of low-pass filtering could also be attained with this circuit simply by adding a capacitor in parallel with the feedback resis- tor as just shown in Figure 1. Figure 3. External Op Amp Connections Residual DAC differential linearity error around midscale can be externally trimmed out, improving THD beyond the AD766’s guaranteed tested specifications. This error is most significant with low-amplitude signals because the ratio of the midscale linearity error to the signal amplitude is greatest in this case, thereby increasing THD. The MSB adjust circuitry shown in Figure 4 can be used for improving THD with low-level sig- nals. Otherwise, the AD766 will operate to its specifications with MSB ADJ (Pin 14) and TRIM (Pin 15) unconnected. Figure 4. Optional MSB Adjustment Circuit ANALOG CIRCUIT CONSIDERATIONS GROUNDING RECOMMENDATIONS The AD766 has two ground pins, designated AGND (analog ground) and DGND (digital ground). The analog ground pin is the “high-quality” ground reference point for the device. The analog ground pin should be connected to the analog common point in the system. The output load should also be connected to that same point. Analog Circuit Considerations–AD766 |
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