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AD678TD Folha de dados(PDF) 11 Page - Analog Devices |
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AD678TD Folha de dados(HTML) 11 Page - Analog Devices |
11 / 14 page AD678 REV. C –11– The AD678 incorporates several features to help the user’s layout. 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 mini- mizes input trace impedance errors. Finally, ground currents have been minimized by careful circuit design. Current through AGND is 200 µA, with no code-dependent variation. The cur- rent through DGND is dominated by the return current for DB11–DB0 and EOC. SUPPLY DECOUPLING The AD678 power supplies should be well filtered, well regulated, and free from high-frequency noise. Switching power supplies are not recommended. These supplies generate spikes which can induce noise in the analog system. Decoupling capacitors should be located as close as possible to all power supply pins. A 10 µF tantalum capacitor in parallel with a 0.1 µF ceramic provides adequate decoupling. The power supply pins should be decoupled directly to AGND. 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 AD678, associated analog input circuitry and interconnec- tions as far as possible from logic circuitry. A solid analog ground plane around the AD678 will isolate large switching ground currents. For these reasons, the use of wire wrap circuit con- struction is not recommended; careful printed circuit construction is preferred. GROUNDING If a single AD678 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 AD678. If multiple AD678s are used or the AD678 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. INTERFACING THE AD678 TO MICROPROCESSORS The I/O capabilities of the AD678 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. The following examples illustrate typical AD678 interface configurations. A single-pass calibration can be done by substituting a bipolar offset trim (error at minus full scale) for the bipolar zero trim (error at midscale), using the same circuit. First, apply a signal 1/2 LSB above minus full scale (–4.9988 V for a ±5 V range) and adjust R1 until the minus full-scale transition is located (1000 0000 0000 to 1000 0000 0001). Then perform the gain error trim as outlined above. Figure 12. Unipolar Input Connections with Gain and Offset Trims Figure 13. Bipolar Input Connections with Gain and Offset Trims BOARD LAYOUT Designing with high-resolution data converters requires careful attention to layout. Trace impedance is a significant issue. At the 12-bit level, a 5 mA current through a 0.5 Ω trace will develop a voltage drop of 2.5 mV, which is 1 LSB for a 10 V full-scale span. In addition to ground drops, inductive and capacitive coupling need to be considered, especially when high- accuracy analog signals share the same board with digital signals. Finally, power supplies need to be decoupled in order to filter out ac noise. Analog and digital signals should not share a common path. Each signal should have an appropriate analog or digital return routed close to it. Using this approach, signal loops enclose a small area, minimizing the inductive coupling of noise. Wide PC tracks, large gauge wire, and ground planes are highly recom- mended to provide low impedance signal paths. Separate analog 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. |
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