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AD7674 Datasheet(PDF) 25 Page - Analog Devices |
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AD7674 Datasheet(HTML) 25 Page - Analog Devices |
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25 / 28 page  AD7674 Rev. A | Page 25 of 28 APPLICATION HINTS LAYOUT The AD7674 has very good immunity to noise on the power supplies. However, care should still be taken with regard to grounding layout. The printed circuit board that houses the AD7674 should be designed so that the analog and digital sections are separated and confined to certain areas of the board. This calls for the use of ground planes, which can be easily separated. Digital and analog ground planes should be joined in only one place, preferably underneath the AD7674, or at least as close to the AD7674 as possible. If the AD7674 is in a system where multiple devices require analog-to-digital ground connections, the connection should still be made at one point only, a star ground point that should be established as close to the AD7674 as possible. The user should avoid running digital lines under the device, as these will couple noise onto the die. The analog ground plane should be allowed to run under the AD7674 to avoid noise coupling. Fast switching signals like CNVST or clocks should be shielded with digital ground to avoid radiating noise to other sections of the board, and should never run near analog signal paths. Crossover of digital and analog signals should be avoided. Traces on different but close layers of the board should run at right angles to each other. This will reduce the effect of feedthrough through the board. The power supply lines to the AD7674 should use as large a trace as possible to provide low impedance paths and reduce the effect of glitches on the power supply lines. Good decoupling is also important to lower the supply’s impedance presented to the AD7674 and to reduce the magnitude of the supply spikes. Decoupling ceramic capacitors, typically 100 nF, should be placed close to and ideally right up against each power supply pin (AVDD, DVDD, and OVDD) and their corresponding ground pins. Additionally, low ESR 10 μF capacitors should be located near the ADC to further reduce low frequency ripple. The DVDD supply of the AD7674 can be a separate supply or can come from the analog supply, AVDD, or the digital interface supply, OVDD. When the system digital supply is noisy or when fast switching digital signals are present, and if no separate supply is available, the user should connect the DVDD digital supply to the analog supply AVDD through an RC filter, (see Figure 27), and connect the system supply to the interface digital supply OVDD and the remaining digital circuitry. When DVDD is powered from the system supply, it is useful to insert a bead to further reduce high frequency spikes. The AD7674 has four different ground pins: REFGND, AGND, DGND, and OGND. REFGND senses the reference voltage and should be a low impedance return to the reference because it carries pulsed currents. AGND is the ground to which most internal ADC analog signals are referenced. This ground must be connected with the least resistance to the analog ground plane. DGND must be tied to the analog or digital ground plane depending on the configuration. OGND is connected to the digital system ground. The layout of the decoupling of the reference voltage is important. The decoupling capacitor should be close to the ADC and should be connected with short and large traces to minimize parasitic inductances. EVALUATING THE AD7674’S PERFORMANCE An evaluation board for the AD7674 allows a quick means to measure both DC (histograms and time domain) and AC (time and frequency domain) performances of the converter. The EVAL-AD7674CBZ is an evaluation board package that includes a fully assembled and tested evaluation board, documentation, and software. The accompanying software requires the use of a capture board which must be ordered seperately from the evaluation board (see the Ordering Guide for information). The evaluation board can also be used in a stand alone configuration and does not use the software when in this mode. Refer to the EVAL-AD76XXEDZ and EVAL- AD76XXCBZ documents available from www.analog.com for evaluation board details. Two types of data capture boards can be used with the EVAL- AD7674CBZ: • USB based (EVAL-CED1Z recommended) • Parallel port based (EVAL-CONTROL BRD3Z not recommended as many newer PC’s do not include parallel ports any longer) The recommended board layout for the AD7674 is outlined in the evaluation board data sheet. |
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