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DAC708 数据表(PDF) 9 Page - Texas Instruments |
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DAC708 数据表(HTML) 9 Page - Texas Instruments |
9 / 13 page 9 ® DAC707/708/709 In Figures 7 and 8, lead and contact resistances are repre- sented by R 1 through R5. As long as the load resistance RL is constant, R 2 simply introduces a gain error and can be removed with gain calibration. R 3 is part of RL if the output voltage is sensed at ANALOG COMMON. Figures 8 and 9 show two methods of connecting the current output model with an external precision output op amp. By sensing the output voltage at the load resistor (connecting R F to the output of the amplifier at R L) the effect of R1 and R2 is greatly reduced. R 1 will cause a gain error but is indepen- dent of the value of R L and can be eliminated by initial calibration adjustments. The effect of R 2 is negligible be- cause it is inside the feedback loop of the output op amp and is therefore greatly reduced by the loop gain. In many applications it is impractical to sense the output voltage at ANALOG COMMON. Sensing the output volt- age at the system ground point is permissible because these converters have separate analog and digital common lines and the analog return current is a near-constant 2mA and varies by only 10 µA to 20µA over the entire input code range. R 4 can be as large as 3 Ω without adversely affecting the linearity of the D/A converter. The voltage drop across R 4 is constant and appears as a zero error that can be nulled with the zero calibration adjustment. Another approach senses the output at the load as shown in Figure 9. In this circuit the output voltage is sensed at the load common and not at the D/A converter common as in the previous circuits. The value of R 6 and R7 must be adjusted for maximum common-mode rejection across R L. The effect of R 4 is negligible as explained previously. The D/A converter and the wiring to its connectors should be located to provide optimum isolation from sources of RFI and EMI. The key to elimination of RF radiation or pickup is small loop area. Signal leads and their return conductors should be kept close together such that they present a small flux-capture cross section for any external field. FIGURE 9. Alternate Connection for Ground Sensing at the Load (Current Output Models). 5 6 7 DAC RF R 1 R 2 3 R R R R R Sense Output To System Ground L R DAC708 Ω 4k Ω 2k Ω 10k 2mA +1% 0 to 2mA Micro- Processor Interface RF L 2 CC DD 4 3 R R R R 1µF + 1µF + 1µF + +V V ±V CC Digital Common Analog Common Alternate Ground Sense Connection System Ground DD V –VCC Supply Supply Sense Output Digital Common Analog Common DAC707/709 FIGURE 7. DAC707/709 Bipolar Output Circuit (Voltage Out). 1 R Ω 2.45k Ω 10k Micro- Processor Interface RF L 2 CC DD 3 R R R 1µF + 1µF + 1µF + +V V ±V CC Digital Common Analog Common Alternate Ground Sense Connection System Ground DD V –VCC Supply Supply Sense Output Digital Common Analog Common B R 1 RF IOUT DAC708 4 R FIGURE 8. DAC708 Bipolar Output Circuit (with External Op Amp). |
类似零件编号 - DAC708 |
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类似说明 - DAC708 |
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