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AD7278BRMZ Datasheet(PDF) 25 Page - Analog Devices |
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AD7278BRMZ Datasheet(HTML) 25 Page - Analog Devices |
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25 / 28 page 
Data Sheet AD7276/AD7277/AD7278 Rev. D | Page 25 of 28 APPLICATION HINTS GROUNDING AND LAYOUT The printed circuit board that houses the AD7276/AD7277/ AD7278 should be designed so that the analog and digital sections are separated and confined to certain areas of the board. This design facilitates using ground planes that can easily be separated. To provide optimum shielding for ground planes, a minimum etch technique is generally best. All AGND pins of the AD7276/ AD7277/AD7278 should be sunk into the AGND plane. Digital and analog ground planes should be joined in one place only. If the AD7276/AD7277/AD7278 are in a system where multiple devices require an AGND-to-DGND connection, the connection should still be made at only one point, a star ground point established as close as possible to the ground pin on the AD7276/AD7277/AD7278. Avoid running digital lines under the device because this couples noise onto the die. However, the analog ground plane should be allowed to run under the AD7276/AD7277/AD7278 to avoid noise coupling. The power supply lines to the AD7276/ AD7277/AD7278 should use as large a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. To avoid radiating noise to other sections of the board, components with fast-switching signals, such as clocks, should be shielded with digital ground, and they should never be run near the analog inputs. Avoid crossover of digital and analog signals. To reduce the effects of feedthrough within the board, traces on opposite sides of the board should run at right angles to each other. A microstrip technique is by far the best method, but it is not always possible to use this approach with a double- sided board. In this technique, the component side of the board is dedicated to ground planes, and signals are placed on the solder side. Good decoupling is also important. All analog supplies should be decoupled with 10 µF ceramic capacitors in parallel with 0.1 µF capacitors to GND. To achieve the best results from these decoupling components, they must be placed as close as possible to the device, ideally right up against the device. The 0.1 µF capacitors should have low effective series resistance (ESR) and low effective series inductance (ESI), such as is typical of common ceramic or surface-mount types of capacitors. Capacitors with low ESR and low ESI provide a low impedance path to ground at high frequencies, which allow them to handle transient currents due to internal logic switching. EVALUATING PERFORMANCE The recommended layout for the AD7276/AD7277/AD7278 is outlined in the evaluation board documentation. The evaluation board package includes a fully assembled and tested evaluation board, documentation, and software for controlling the board from the PC via the evaluation board controller. To demonstrate/ evaluate the ac and dc performance of the AD7276/AD7277, the evaluation board controller can be used in conjunction with the AD7276/AD7277 evaluation board, as well as with many other Analog Devices evaluation boards ending in the CB designator, The software allows the user to perform ac (fast Fourier transform) and dc (histogram of codes) tests on the AD7276/ AD7277. The software and documentation are on a CD shipped with the evaluation board. |
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