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AD7715 Datasheet(PDF) 29 Page - Analog Devices |
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AD7715 Datasheet(HTML) 29 Page - Analog Devices |
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29 / 41 page  AD7715 Data Sheet Rev. E | Page 28 of 40 POWER SUPPLIES There is no specific power sequence required for the AD7715; either the AVDD or the DVDD supply can come up first. While the latch-up performance of the AD7715 is good, it is important that power is applied to the AD7715 before signals at REF IN, AIN, or the logic input pins to avoid excessive currents. If this is not possible, then the current that flows in any of these pins should be limited. If separate supplies are used for the AD7715 and the system digital circuitry, then the AD7715 should be powered up first. If it is not possible to guarantee this, then current limiting resistors should be placed in series with the logic inputs to again limit the current. During normal operation the AD7715 analog supply (AVDD) should always be greater than or equal to its digital supply (DVDD). Supply Current The current consumption on the AD7715 is specified for supplies in the range 3 V to 3.6 V and in the range 4.75 V to 5.25 V. The part operates over a 2.85 V to 5.25 V supply range and the IDD for the part varies as the supply voltage varies over this range. Figure 7 shows the variation of the typical IDD with VDD voltage for both a 1 MHz external clock and a 2.4576 MHz external clock at 25°C. The AD7715 is operated in unbuffered mode. The relationship shows that the IDD is minimized by operating the part with lower VDD voltages. IDD on the AD7715 is also minimized by using an external master clock or by optimizing external components when using the on-chip oscillator circuit. SUPPLY VOLTAGE, AVDD AND DVDD (V) 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 7. IDD vs. Supply Voltage Grounding and Layout Because 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 removes common-mode noise on these inputs. The analog and digital supplies to the AD7715 are independent and separately pinned out to minimize coupling between the analog and digital sections of the device. The digital filter provides 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 AD7715 is more immune to noise interference than a conventional high resolution converter. However, because the resolution of the AD7715 is so high and the noise levels from the AD7715 so low, care must be taken with regard to grounding and layout. The printed circuit board that houses the AD7715 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 AD7715 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 AD7715. If the AD7715 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 AD7715. Avoid running digital lines under the device as these couples noise onto the die. The analog ground plane should be allowed to run under the AD7715 to avoid noise coupling. The power supply lines to the AD7715 should use as large 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 reduces 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 must 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 AD7715, it is recommended that the AVDD supply of the system is used. This supply should have the recommended analog supply decoupling capacitors between the AVDD pin of the AD7715 and AGND and the recommended digital supply decoupling capacitor between the DVDD pin of the AD7715 and DGND. |
Similar Part No. - AD7715_17 |
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Similar Description - AD7715_17 |
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