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AD677JR датащи(PDF) 10 Page - Analog Devices |
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AD677JR датащи(HTML) 10 Page - Analog Devices |
10 / 16 page AD677 REV. A –10– 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 capaci- tive coupling need to be considered, especially when high accu- racy 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 AD677 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 AD677 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 AD677 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. Using AGND SENSE to remotely sense the ground potential of the signal source can be useful if the signal has to be carried some distance to the A/D converter. Since all IC ground cur- rents have to return to the power supply and no ground leads are free from resistance and inductance, there are always some voltage differences from one ground point in a system to another. Over distance this voltage difference can easily amount to sev- eral LSBs (in a 10 V input span, 16-bit system each LSB is about 0.15 mV). This would directly corrupt the A/D input sig- nal if the A/D measures its input with respect to power ground (AGND) as shown in Figure 5a. To solve this problem the AD677 offers an AGND SENSE pin. Figure 5b shows how the AGND SENSE can be used to eliminate the problem in Figure 5a. Figure 5b also shows how the signal wires should be VIN AGND SOURCE VS GROUND LEAD IGROUND > 0 TO POWER SUPPLY GND AD677 ∆V Figure 5a. Input to the A/D is Corrupted by IR Drop in Ground Leads: VIN = VS + ∆V. VIN AGND SENSE AGND SOURCE VS SHIELDED CABLE GROUND LEAD I GROUND > 0 TO POWER SUPPLY GND AD677 Figure 5b. AGND SENSE Eliminates the Problem in Figure 5a. shielded in a noisy environment to avoid capacitive coupling. If inductive (magnetic) coupling is expected to be dominant such as where motors are present, twisted-pair wires should be used instead. The digital ground pin is the reference point for all of the digital signals that operate the AD677. This pin should be connected to the digital common point in the system. As Figure 4 illus- trated, the analog and digital grounds should be connected together at one point in the system, preferably at the AD677. VOLTAGE REFERENCE The AD677 requires the use of an external voltage reference. The input voltage range is determined by the value of the refer- ence voltage; in general, a reference voltage of n volts allows an input range of ±n volts. The AD677 is specified for a voltage reference between +5 V and +10 V. A 10 V reference will typi- cally require support circuitry operated from ±15 V supplies; a 5.0 V reference may be used with ±12 V supplies. Signal-to- noise performance is increased proportionately with input signal range (see Figure 12). In the presence of a fixed amount of sys- tem noise, increasing the LSB size (which results from increas- ing the reference voltage) will increase the effective S/(N+D) performance. Figure 11 illustrates S/(N+D) as a function of ref- erence voltage. In contrast, dc accuracy will be optimal at lower reference voltage values (such as 5 V) due to capacitor nonlin- earity at higher voltage values. During a conversion, the switched capacitor array of the AD677 presents a dynamically changing current load at the voltage ref- erence as the successive-approximation algorithm cycles through various choices of capacitor weighting. (See the following sec- tion “Analog Input” for a detailed discussion of the VREF input characteristics.) The output impedance of the reference circuitry must be low so that the output voltage will remain sufficiently constant as the current drive changes. In some applications, this may require that the output of the voltage reference be buffered by an amplifier with low impedance at relatively high frequen- cies. In choosing a voltage reference, consideration should be |
Аналогичный номер детали - AD677JR |
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Аналогичное описание - AD677JR |
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