DAC_current_loop_controller

Published:2009/7/25 5:40:00 Author:Jessie | From:SeekIC

Figure 9-41 shows a DAC0830 connected to provide digital control ofthe standard 3-mA to 20 mA industrial-process current loop. The circuit is two-terminal, and all circuit components (including the DAC) are powered directly from the loop，The output transistor conducts whatever current is necessary to keep the voltage across R3 equal to the voltage across R2．This voltage and therefore the total loop current is directly proportional to the output current from the DAC. The net resistance of R1 is used to set the zero-code loop current to 4 mA and R2 is adjusted to provide the 16-mA output span for a full-scale DAC code. The entire circuit floats by operating whatever ground-reference potential is required for the total loop resistance andloop current，The voltage differential between the input and output terminals mustbe kept in the range of 16 V to 55 V and the digital inputs to the DAC must beelectronically isolated from the ground potential. of the controlling microprocessor．This isolation can best be achieved with opto-isolators．in a non microprocessor system in which the loop-controlling information comes from thumb wheel switches(or a similar mechanical device) the digital input for the DAC can be taken from BCD-to-binary CMOS logic elements (which are ground referenced to the groundpotential of the DAC)．The total Supply current requirements of all circuits usedmust (of course) be less than 4 mA and R1 can be adjusted accordingly． NATIONAL SEMICONDUCTOR， APPLICATION NOTE 271. 1994. P. 670.



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