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AD7783BRU датащи(PDF) 9 Page - Analog Devices |
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AD7783BRU датащи(HTML) 9 Page - Analog Devices |
9 / 12 page REV. B AD7783 –9– to the voltage on the AIN(–) input. For example, if AIN(–) is 2.5 V and the AD7783 is configured for an analog input range of ±160 mV, the analog input range on the AIN(+) input is 2.34 V to 2.66 V (i.e., 2.5 V ± 0.16 V). The coding is offset binary with a negative full-scale voltage resulting in a code of 000 . . . 000, a zero differential voltage resulting in a code of 100 . . . 000, and a positive full-scale voltage resulting in a code of 111 . . . 111. The output code for any analog input voltage can be represented as follows: Code AIN GAIN V N REF =¥ ¥ ¥ () ()+ [] - 21 024 1 1 /. where AIN is the analog input voltage, GAIN is the PGA gain, i.e., 1 on the ±2.56 V range and 16 on the ±160 mV range, and N = 24. Excitation Currents The AD7783 also contains two matched 200 mA constant cur- rent sources. Both source current from VDD that is directed to either the IOUT1 or IOUT2 pins of the device depending on the polarity of the IPIN pin. These current sources can be used to excite external resistive bridge or RTD sensors. Crystal Oscillator The AD7783 is intended for use with a 32.768 kHz watch crys- tal. A PLL internally locks onto a multiple of this frequency to provide a stable 4.194304 MHz clock for the ADC. The modu- lator sample rate is the same as the crystal oscillator frequency. The start-up time associated with 32.768 kHz crystals is typically 300 ms. In some cases, it will be necessary to connect capacitors on the crystal to ensure that it does not oscillate at overtones of its fundamental operating frequency. The values of capacitors will vary depending on the manufacturer’s specifications. Reference Input The AD7783 has a fully differential reference input capability for the channel. The common-mode range for differential inputs is from GND to VDD. The reference input is unbuffered, and therefore excessive R-C source impedances will introduce gain errors. The reference voltage REFIN (REFIN(+) – REFIN(–)) is 2.5 V nominal for specified operation, but the AD7783 is functional with reference voltages from 1 V to VDD. In applica- tions where the excitation (voltage or current) for the transducer on the analog input also drives the reference voltage for the part, the effect of the low frequency noise in the excitation source will be removed as the application is ratiometric. If the AD7783 is used in a nonratiometric application, a low noise reference should be used. Recommended reference voltage sources for the AD7783 include the AD780, REF43, and REF192. It should also be noted that the reference inputs provide a high impedance, dynamic load. Because the input impedance of each reference input is dynamic, resistor/capacitor combinations on these inputs can cause dc gain errors, depending on the output impedance of the source that is driving the reference inputs. Recommended reference voltage sources (e.g., AD780) will typically have low output impedances and are, therefore, tolerant to having decoupling capacitors on the REFIN(+) without introducing gain errors in the system. Deriving the reference input voltage across an external resistor will mean that the reference input sees a significant external source impedance. External decoupling on the REFIN pins would not be recommended in this type of circuit configuration. Grounding and Layout Since the analog inputs and reference inputs on the ADC are differential, most of the voltages in the analog modulator are common-mode voltages. The excellent common-mode rejection of the part will remove common-mode noise on these inputs. The digital filter will provide rejection of broadband noise on the power supply, except at integer multiples of the modulator sampling frequency. The digital filter also removes noise from the analog and reference inputs, provided these noise sources do not saturate the analog modulator. As a result, the AD7783 is more immune to noise interference than a conventional high resolution converter. However, because the resolution of the AD7783 is so high, and the noise levels from the AD7783 are so low, care must be taken with regard to grounding and layout. The printed circuit board that houses the AD7783 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. A minimum etch technique is generally best for ground planes as it gives the best shielding. It is recommended that the AD7783’s GND pin be tied to the AGND plane of the system. In any layout, it is important that the user keep in mind the flow of currents in the system, ensur- ing that the return paths for all currents are as close as possible to the paths the currents took to reach their destinations. Avoid forcing digital currents to flow through the AGND sections of the layout. The AD7783’s ground plane should be allowed to run under the AD7783 to prevent noise coupling. The power supply lines to the AD7783 should use as wide a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching signals like clocks should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never be run near the analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This will reduce the effects of feedthrough through the board. A microstrip technique is by far the best but is not always possible with a double-sided board. In this technique, the component side of the board is dedicated to ground planes while signals are placed on the solder side. Good decoupling is important when using high resolution ADCs. VDD should be decoupled with 10 mF tantalum in parallel with 0.1 mF capacitors to GND. To achieve the best from these decoupling components, they have to be placed as close as pos- sible to the device, ideally right up against the device. All logic chips should be decoupled with 0.1 mF ceramic capaci- tors to DGND. |
Аналогичный номер детали - AD7783BRU |
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Аналогичное описание - AD7783BRU |
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