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AD7841BS Datasheet(PDF) 10 Page - Analog Devices |
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AD7841BS Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page AD7841 –10– REV. 0 DUTGND Voltage Range During power-on, the VOUT pins of the AD7841 are connected to the relevant DUTGND pins via G5 and the 14 k Ω thin-film resistor. The DUTGND potential must obey the max ratings at all times. Thus, the voltage at DUTGND must always be within the range VSS – 0.3 V, VDD + 0.3 V. However, in order that the voltages at the VOUT pins of the AD7841 stay within ± 2 V of the relevant DUTGND potential during power-on, the voltage applied to DUTGND should also be kept within the range GND – 2 V, GND + 2 V. Once the AD7841 has powered on and the on-chip amplifiers have settled, any voltage that is now applied to the DUTGND pin is subtracted from the DAC output, which has been gained up by a factor of two. Thus, for specified operation, the maxi- mum voltage that can be applied to the DUTGND pin in- creases to the maximum allowable 2 VREF(+) voltage, and the minimum voltage that can be applied to DUTGND is the minimum 2 VREF(–) voltage. After the AD7841 has fully powered on, the outputs can track any DUTGND voltage within this minimum/maximum range. Power Supply Sequencing When operating the AD7841, it is important that ground be connected at all times to avoid high current states. The recom- mended power-up sequence is VDD/VSS followed by VCC. If VCC can exceed VDD on power-up, the diode scheme shown in the absolute maximum ratings section will ensure protection. The reference inputs and digital inputs should be powered up last. Should the references exceed VDD/VSS on power-up, current limiting resistors should be inserted in series with the reference inputs to limit the current to 20 mA. Logic inputs should not be applied before VCC. Current limiting resistors (470 Ω) in series with the logic inputs should be inserted if these inputs come up before VCC. MICROPROCESSOR INTERFACING Interfacing the AD7841—16-Bit Interface The AD7841 can be interfaced to a variety of 16-bit micro- controllers or DSP processors. Figure 19 shows the AD7841 interfaced to a generic 16-bit microcontroller/DSP processor. The lower address lines from the processor are connected to A0, A1 and A2 on the AD7841 as shown. The upper address lines are decoded to provide a chip select signal or an LDAC signal for the AD7841. The fast interface timing of the AD7841 allows direct interface to a wide variety of microcontrollers and DSPs as shown in Figure 19. AD7841 CONTROLLER/ DSP PROCESSOR* ADDRESS DECODE D13 D0 DATA BUS UPPER BITS OF ADDRESS BUS A2 A1 A0 R/ W *ADDITIONAL PINS OMITTED FOR CLARITY D13 D0 CS LDAC A2 A1 A0 WR Figure 19. AD7841 Parallel Interface APPLICATIONS Power Supply Bypassing and Grounding In any circuit where accuracy is important, careful consideration of the power supply and ground return layout helps to ensure the rated performance. The printed circuit board on which the AD7841 is mounted 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 that can be easily separated. A minimum etch technique is generally best for ground planes as it gives the best shielding. Digital and ana- log ground planes should be joined at only one place. The GND pin of the AD7841 should be connected to the AGND of the system. If the AD7841 is in a system where multiple devices require an AGND-to-DGND connection, the connection should be made at one point only, a star ground point that should be established as close as possible to the AD7841. Digital lines running under the device should be avoided as these will couple noise onto the die. The analog ground plane should be allowed to run under the AD7841 to avoid noise coupling. The power supply lines of the AD7841 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 parts of the board and 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 micro- strip technique is by far the best but not always possible with a double sided board. In this technique, the component side of the board is dedicated to ground plane while signal traces are placed on the solder side. The AD7841 should have ample supply bypassing located as close to the package as possible, ideally right up against the |
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