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LTC1453 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1453 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC1451 LTC1452/LTC1453 sn145123 145123fas An Isolated 4mA to 20mA Process Controller Has 3.3V Minimum Loop Voltage 11451/2/3 TA04 3k 10k 1k 45k 5k 90k 5k Q1 2N3440 RS 10 Ω VLOOP 3.3V TO 30V IOUT OUT IN CLK DIN CS/LD CLK DIN CS/LD CLK DIN CS/LD VCC VOUT 1 µF LTC1453 4N28 OPTO-ISOLATORS 3.3V 500 Ω LT ®1121-3.3 FROM OPTO- ISOLATED INPUTS VREF + – LT1077 This circuit shows how to use an LTC1453 to make an opto-isolated digitally controlled 4mA to 20mA process controller. The controller circuitry, including the opto- isolation, is powered by the loop voltage that can have a wide range of 3.3V to 30V. The 1.22V reference output of the LTC1453 is used for the 4mA offset current and VOUT is used for the digitally controlled 0mA to 16mA current. RS is a sense resistor and the op amp modulates the transistor Q1 to provide the 4mA to 20mA current through this resistor. The potentiometers allow for offset and full- scale adjustment. The control circuitry dissipates well under the 4mA budget at zero-scale. Note that although these DACs have internal Schmitt triggers and are suitable for use with slow rising edges such as produced by the above optoisolator, the use of optoisolators in a daisy-chained topology requires the addition of a gate or the use of a fast isolator on the clock signal. Setup and hold times between DOUT and DIN are not guaranteed unless a clock edge with a rise time of less than 100ns is provided. TYPICAL APPLICATIO S |
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Similar Description - LTC1453 |
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