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AD7714ARZ-5 データシート(PDF) 28 Page - Analog Devices |
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AD7714ARZ-5 データシート(HTML) 28 Page - Analog Devices |
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28 / 40 page  AD7714 REV. C –28– SUPPLY VOLTAGE (AVDD & DVDD) – Volts 0 2.85 0.9 1.0 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 3.15 3.45 4.05 4.35 4.65 4.95 5.25 3.75 MCLK IN = 2.4576MHz MCLK IN = 1MHz Figure 5. IDD vs. Supply Voltage Grounding and Layout Since the analog inputs and reference input 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 analog and digital supplies to the AD7714 are independent and sepa- rately pinned out to minimize coupling between the analog and digital sections of the device. The digital filter will provide rejection of broadband noise on the power supplies, except at integer multiples of the modulator sampling frequency. The digital filter also removes noise from the analog and reference inputs provided those noise sources do not saturate the analog modulator. As a result, the AD7714 is more immune to noise interference that a conventional high resolution converter. How- ever, because the resolution of the AD7714 is so high and the noise levels from the AD7714 so low, care must be taken with regard to grounding and layout. The printed circuit board which houses the AD7714 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. This facilitates the use of ground planes which can be separated easily. A minimum etch technique is generally best for ground planes as it gives the best shielding. Digital and analog ground planes should only be joined in one place. If the AD7714 is the only device requiring an AGND to DGND connection, then the ground planes should be connected at the AGND and DGND pins of the AD7714. If the AD7714 is in a system where multiple devices require AGND to DGND connections, the connection should still be made at one point only, a star ground point which should be established as close as possible to the AD7714. 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 AD7714 to avoid noise coupling. The power supply lines to the AD7714 should use as large a trace as pos- sible 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. All analog supplies should be decoupled with 10 µF tantalum in parallel with 0.1 µF capacitors to AGND. To achieve the best from these decoupling components, they have to be placed as close as possible to the device, ideally right up against the device. All logic chips should be decoupled with 0.1 µF disc ceramic capacitors to DGND. In systems where a common supply voltage is used to drive both the AVDD and DVDD of the AD7714, it is recommended that the system’s AVDD supply is used. This supply should have the recommended analog supply decoupling capacitors between the AVDD pin of the AD7714 and AGND and the recommended digital supply decoupling capacitor between the DVDD pin of the AD7714 and DGND. Evaluating the AD7714 Performance The recommended layout for the AD7714 is outlined in the evaluation board for the AD7714. The evaluation board pack- age includes a fully assembled and tested evaluation board, documentation, software for controlling the board over the printer port of a PC and software for analyzing the AD7714’s performance on the PC. For the AD7714-5, the evaluation board order number is EVAL-AD7714-5EB and for the AD7714-3, the order number is EVAL-AD7714-3EB. Noise levels in the signals applied to the AD7714 may also affect performance of the part. The AD7714 allows two tech- niques for evaluating the true performance of the part, indepen- dent of the analog input signal. These schemes should be used after a calibration has been performed on the part. The first of these is to select the AIN6/AIN6 input channel arrangement. In this case, the differential inputs to the AD7714 are internally shorted together to provide a zero differential voltage for the analog modulator. External to the device, the AIN6 input should be connected to a voltage that is within the allowable common-mode range of the part. The second scheme is to evaluate the part with a voltage near the input full scale voltage for a gain of 1. To do this, the refer- ence voltage for the part should be applied to the analog input. This will give a fixed full-scale reading from the device. If the zero-scale calibration coefficient is now read from the device, increased by a number equivalent to about 200 decimal and this value reloaded to the zero-scale calibration register, the input range will be offset such that a voltage equal to reference voltage no longer corresponds to a full-scale reading. This allows the user to evaluate the noise performance of the part with a near full-scale voltage. |
同様の部品番号 - AD7714ARZ-5 |
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同様の説明 - AD7714ARZ-5 |
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