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AD1851 Datenblatt(PDF) 6 Page - Analog Devices |
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AD1851 Datenblatt(HTML) 6 Page - Analog Devices |
6 / 12 page AD1851/AD1861 REV. A –6– Analog Circuit Considerations GROUNDING RECOMMENDATIONS The AD1851/AD1861 has two ground pins, designated Analog and Digital ground. The analog ground pin is the “high quality” ground reference point for the device. The analog ground pin should be connected to the analog common point in the system. The output load should also be connected to that same point. The digital ground pin returns ground current from the digital logic portions of the AD1851/AD1861 circuitry. This pin should be connected to the digital common point in the system. As illustrated in Figure 2, the analog and digital grounds should be connected together at one point in the system. DGND AGND 5V + AD1851/AD1861 5V – ANALOG GROUND DIGITAL GROUND 3 16 1 2 12 +5V +VL +VS –VS Figure 2. Recommended Circuit Schematic POWER SUPPLIES AND DECOUPLING The AD1851/AD1861 has three power supply input pins. The ±V S supplies provide the supply voltages to operate the linear portions of the DAC including the voltage reference, output am- plifier and control amplifier. The ±V S supplies are designed to operate at ±5 V. The +VL supply operates the digital portions of the chip includ- ing the input shift register and the input latching circuitry. The +VL supply is designed to operate at +5 V. Decoupling capacitors should be used on all power supply pins. Furthermore, good engineering practice suggests that these ca- pacitors be placed as close as possible to the package pins as well as to the common points. The logic supply, +VL, should be decoupled to digital common, while the analog supplies, ±V S, should be decoupled to analog common. The use of three separate power supplies will reduce feedthrough from the digital portion of the system to the linear portion of the system, thus contributing to improved performance. However, three separate voltage supplies are not necessary for good circuit performance. For example, Figure 3 illustrates a system where only a single positive and a single negative supply are available. In this example, the positive logic and positive analog supplies must both be connected to +5 V, while the negative analog sup- ply will be connected to –5 V. Performance would benefit from a measure of isolation between the supplies introduced by using simple low pass filters in the individual power supply leads. +VL DGND AGND AD1851/AD1861 ANALOG GROUND DIGITAL GROUND 3 16 1 2 12 +VS –VS –5V +5V +5V Figure 3. Alternate Recommended Schematic As with most linear circuits, changes in the power supplies will affect the output of the DAC. Analog Devices recommends that well regulated power supplies with less than 1% ripple be incor- porated into the design of any system using the AD1851/AD1861. OPTIONAL MSB ADJUSTMENT Use of an optional adjustment circuit allows residual differential linearity error around midscale to be eliminated. This error is especially important when low amplitude signals are being re- produced. In those cases, as the signal amplitude decreases, the ratio of the midscale differential linearity error to the signal am- plitude increases, thereby increasing THD. Therefore, for best performance at low output levels, the op- tional MSB adjust circuitry shown in Figure 4 may be used to improve performance. The adjustment should be made with a small signal input (–20 dB or –60 dB). 15 14 1 TRIM 470k Ω 100k Ω 200k Ω MSB ADJUST –VS Figure 4. Optional THD Adjust Circuit |
Ähnliche Teilenummer - AD1851 |
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Ähnliche Beschreibung - AD1851 |
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