LTC1451 Ver la hoja de datos (PDF) - Linear Technology
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LTC1451 Datasheet PDF : 12 Pages
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TYPICAL APPLICATIO S
LTC1451
LTC1452/LTC1453
An Isolated 4mA to 20mA Process Controller
Has 3.3V Minimum Loop Voltage
LT®1121-3.3
IN OUT
1µF
FROM
OPTO-
ISOLATED
INPUTS
90k 5k
VCC
VREF
CLK
45k 5k
DIN LTC1453 VOUT
+
1k
CS/LD
LT1077
3k
–
VLOOP
3.3V TO 30V
Q1
2N3440
CLK
DIN
CS/LD
OPTO-ISOLATORS
3.3V
10k
500Ω
4N28
CLK
DIN
CS/LD
RS
10Ω
IOUT
11451/2/3 TA04
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.
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