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AD676 bảng dữ liệu(PDF) 11 Page - Analog Devices |
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AD676 bảng dữ liệu(HTML) 11 Page - Analog Devices |
11 / 17 page AD676 REV. A –10– POWER SUPPLIES AND DECOUPLING The AD676 has three power supply input pins. VCC and VEE provide the supply voltages to operate the analog portions of the AD676 including the ADC and sample-hold amplifier (SHA). VDD provides the supply voltage which operates the digital por- tions of the AD676 including the data output buffers and the autocalibration controller. As with most high performance linear circuits, changes in the power supplies can produce undesired changes in the perfor- mance of the circuit. Optimally, well regulated power supplies with less than 1% ripple should be selected. The ac output im- pedance of a power supply is a complex function of frequency, and in general will increase with frequency. In other words, high frequency switching such as that encountered with digital cir- cuitry requires fast transient currents which most power supplies cannot adequately provide. This results in voltage spikes on the supplies. If these spikes exceed the ±5% tolerance of the ±12 V supplies or the ±10% limits of the +5 V supply, ADC perfor- mance will degrade. Additionally, spikes at frequencies higher than 100 kHz will also degrade performance. To compensate for the finite ac output impedance of the supplies, it is necessary to store “reserves” of charge in bypass capacitors. These capacitors can effectively lower the ac impedance presented to the AD676 power inputs which in turn will significantly reduce the magni- tude of the voltage spikes. For bypassing to be effective, certain guidelines should be followed. Decoupling capacitors, typically 0.1 µF, should be placed as closely as possible to each power supply pin of the AD676. It is essential that these capacitors be placed physically close to the IC to minimize the inductance of the PCB trace between the capacitor and the supply pin. The logic supply (VDD) should be decoupled to digital common and the analog supplies (Vcc and VEE) to analog common. The ref- erence input is also considered as a power supply pin in this re- gard and the same decoupling procedures apply. These points are displayed in Figure 4. +5V 12V –12V SYSTEM ANALOG COMMON SYSTEM DIGITAL COMMON AGND DGND AD676 17 12 13 11 18 EE V CC V 11 V REF VDD 0.1 µF 0.1 µF 0.1 µF 0.1 µF Figure 4. Grounding and Decoupling the AD676 Additionally, it is beneficial to have large capacitors (>47 µF) located at the point where the power connects to the PCB with 10 µF capacitors located in the vicinity of the ADC to further reduce low frequency ripple. In systems that will be subjected to particularly harsh environmental noise, additional decoupling may be necessary. RC-filtering on each power supply combined with dedicated voltage regulation can substantially decrease power supply ripple effects (this is further detailed in Figure 7). BOARD LAYOUT Designing with high resolution data converters requires careful attention to board layout. Trace impedance is a significant issue. A 1.22 mA current through a 0.5 Ω trace will develop a voltage drop of 0.6 mV, which is 4 LSBs at the 16-bit level for a 10 V full-scale span. In addition to ground drops, inductive and ca- pacitive coupling need to be considered, especially when high accuracy analog signals share the same board with digital signals. Analog and digital signals should not share a common return path. Each signal should have an appropriate analog or digital return routed close to it. Using this approach, signal loops en- close a small area, minimizing the inductive coupling of noise. Wide PC tracks, large gauge wire, and ground planes are highly recommended to provide low impedance signal paths. Separate analog and digital ground planes are also desirable, with a single interconnection point at the AD676 to minimize interference between analog and digital circuitry. Analog signals should be routed as far as possible from digital signals and should cross them, if at all, only at right angles. A solid analog ground plane around the AD676 will isolate it from large switching ground currents. For these reasons, the use of wire wrap circuit con- struction will not provide adequate performance; careful printed circuit board construction is preferred. GROUNDING The AD676 has three grounding pins, designated ANALOG GROUND (AGND), DIGITAL GROUND (DGND) and ANALOG GROUND SENSE (AGND SENSE). The analog ground pin is the “high quality” ground reference point for the device, and should be connected to the analog common point in the system. AGND SENSE is intended to be connected to the input signal ground reference point. This allows for slight differences in level between the analog ground point in the system and the input signal ground point. However no more than 100 mV is recom- mended between the AGND and the AGND SENSE pins for specified performance. |
Số phần tương tự - AD676_17 |
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Mô tả tương tự - AD676_17 |
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