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AD679KD Datasheet(PDF) 10 Page - Analog Devices |
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AD679KD Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page AD679 REV. C –10– and digital ground planes are also desirable, with a single inter- connection point to minimize ground loops. Analog signals should be routed as far as possible from digital signals and should cross them at right angles. The AD679 incorporates several features to help the user’s lay- out. Analog pins (VEE, AIN, AGND, REFOUT, REFIN, BIPOFF, VCC) are adjacent to help isolate analog from digital signals. In addition, the 10 M Ω input impedance of AIN minimizes input trace impedance errors. Finally, ground currents have been minimized by careful circuit architecture. Current through AGND is 200 µA, with no code dependent variation. The cur- rent through DGND is dominated by the return current for DB7–DB0 and EOC. SUPPLY DECOUPLING The AD679 power supplies should be well filtered, well regu- lated, and free from high frequency noise. Switching power sup- plies are not recommended due to their tendency to generate spikes which can induce noise in the analog system. Decoupling capacitors should be used in very close layout prox- imity between all power supply pins and analog ground. A 10 µF tantalum capacitor in parallel with a 0.1 µF ceramic capacitor provides adequate decoupling. An effort should be made to minimize the trace length between the capacitor leads and the respective converter power supply and common pins. The circuit layout should attempt to locate the AD679, associated analog input circuitry and interconnec- tions as far as possible from logic circuitry. A solid analog ground plane around the AD679 will isolate large switching ground currents. For these reasons, the use of wire wrap circuit construction is not recommended; careful printed circuit con- struction is preferred. GROUNDING If a single AD679 is used with separate analog and digital ground planes, connect the analog ground plane to AGND and the digital ground plane to DGND keeping lead lengths as short as possible. Then connect AGND and DGND together at the AD679. If multiple AD679s are used or the AD679 shares ana- log supplies with other components, connect the analog and digital returns together once at the power supplies rather than at each chip. This prevents large ground loops which inductively couple noise and allow digital currents to flow through the ana- log system. USE OF EXTERNAL VOLTAGE REFERENCE The AD679 features an on-chip voltage reference. For improved gain accuracy over temperature, a high performance external voltage reference may be used in place of the on-chip reference. The AD586 and AD588 are popular references appropriate for use with high resolution converters. The AD586 is a low cost reference which utilizes a buried Zener architecture to provide low noise and drift. The AD588 is a higher performance refer- ence which uses a proprietary implanted buried Zener diode in conjunction with laser-trimmed thin-film resistors for low offset and low drift. Figure 7 shows the use of the AD586 with the AD679 in a bipo- lar input mode. Over the 0 °C to +70°C range, the AD586 L-grade exhibits less than a 2.25 mV output change from its ini- tial value at 25 °C. REF IN (Pin 9) scales its input by a factor of two; thus, this change becomes effectively 4.5 mV. When ap- plied to the AD679, this results in a total gain drift of 0.09% FSR, which is an improvement over the on-chip reference per- formance of 0.11% FSR. A noise-reduction capacitor, CN, has been shown. This capacitor reduces the broadband noise of the AD586 out- put, thereby optimizing the overall ac and dc performance of the AD679. Figure 7. Bipolar Input with Gain and Offset Trims Figure 8 shows the AD679 in unipolar input mode with the AD588 reference. The AD588 output is accurate to 0.65 mV from its value at 25 °C over the 0°C to 70°C range. This results in a 0.06% FSR total gain drift for the AD679, which is a sub- stantial improvement over the on-chip reference performance of 0.11% FSR. A noise-reduction network on Pins 4, 6 and 7 has been shown. The 1 µF capacitors form low pass filters with the internal resistance of the AD588 Zener and amplifier cells and external resistance. This reduces the high frequency (to 1 MHz) noise of the AD588, providing optimum ac and dc performance of the AD679. Figure 8. Unipolar Input with Gain and Offset Trims INTERFACING THE AD679 TO MICROPROCESSORS The I/O capabilities of the AD679 allow direct interfacing to general purpose and DSP microprocessor buses. The asynchro- nous conversion control feature allows complete flexibility and control with minimal external hardware. |
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