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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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DAC729KH-BI Fiches technique(PDF) 8 Page - Burr-Brown (TI) |
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DAC729KH-BI Fiches technique(HTML) 8 Page - Burr-Brown (TI) |
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8 / 12 page  DAC729 8 much more critical as the accuracy of the system increases. The DAC729 has been designed to minimize these applica- tions problems to a large degree. The basics of “Kelvin sensing” and “holy point” grounding will be the most impor- tant considerations in optimizing the absolute accuracy of the system. Figure 9 shows the proper connection of the DAC with the holy-point ground and the Kelvin-sensed- output connection at the load. The DAC729 has three separate supply common (ground) pins. Reference common (pin 33) carries the return current from the internal reference and the output I/V converter common. The current in pin 33 is stable and independent of code or load. Digital common (pin 20) carries the variable currents of the biasing circuits. Analog common (pin 30) is the termination of the R-2R ladder and also carries the “waste current” from the off side of the current switches. These three ground pins must be star connected to system ground for the DAC to bias properly and accurately. Good ground connections are essential, because an IR drop of just 39 µV completely swamps out a 10V FSR 18-bit LSB. When the application is such that the DAC must control loads of greater than ±5mA with rated accuracy, it is recom- mended that an external op amp or op amp buffer combina- tion be used to dissipate the variable power external to the DAC729. This minimizes the temperature variations on the precision D/A converter. Figure 10 illustrates a method of connecting the external amplifier for ±10V operation, while using an external reference. When driving loads to greater than ±10V, care must be taken that the internal resistors are never exposed to greater than ±10V, and that the summing junction is clamped to insure that the voltage never exceeds ±5V. Clamping the summing junction with diodes (parallel opposing connection) to ground will give the best transient response and settling times. TRUE 18-BIT PERFORMANCE (Differential Linearity Adjustment) To take full advantage of the DAC729’s accuracy, the four MSBs have adjustment capabilities. A simplified schematic (Figure 11) shows the internal structure of the DAC current source and the adjustment input terminal. The suggested network for adjusting the linearity is shown in Figure 12. This circuit has nearly twice the range that is required for the DAC729JH. The range is intentionally narrow so as to minimize the effect of temperature drift or stability problems in the potentiometers. The potentiometers are biased in an identical fashion to the internal DAC current sources to minimize power supply sensitivity and drift over tempera- ture. Low leakage capacitors such as Mylar or Teflon film are essential. The linearity adjustment requires a digital voltmeter with 7 digits of resolution on the 10V range (1 µV resolution) and excellent linearity. For the DAC, 1LSB of the 0V to 10V scale (10 FSR) is 38 µV. To be 1/2LSB linear, the measure- ment must resolve 19 µV. The meter must be properly calibrated and linear to 1ppm of range. FIGURE 5. Relationship of Offset and Gain Adjustments for a Unipolar D/A Converter. FIGURE 6. Relationship of Offset and Gain Adjustments for a Bipolar D/A Converter. REFERENCE ADJUSTMENT The internal reference may be fine adjusted using pin 35 as shown in Figure 7. Adjusting the reference has a similar effect on the DAC as gain adjust, except the transfer charac- teristic rotates around bipolar zero for a bipolar connection as shown in Figure 8. LAYOUT/APPLICATIONS SUGGESTIONS Obviously, the management of IR drops, power supply noise, thermal stability, and environmental noise becomes FIGURE 4. Equivalent Resistances. 180k Ω 180k Ω 3.9M Ω 10k Ω 1LSB Full Scale Range Digital Input Gain Adjustment Rotates the Line Range of Gain Adj. Input = 00000 H Offset Adj. Translates the Line Input = 3FFFF H + Full Scale MSB on, All Others Off Range of Gain Adj. 1LSB Bipolar V Offset Digital Input Input = 00000 H Offset Adj. Translates the Line Input = 3FFFF H + Full Scale Full Scale Range Gain Adjustment Rotates the Line Range of Offset Adj. –Full Scale |
Numéro de pièce similaire - DAC729KH-BI |
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Description similaire - DAC729KH-BI |
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