LTC1453 LINER [Linear Technology], LTC1453 Datasheet - Page 9

• Manufacturer Part Number: LTC1453
• Description: 12-Bit Rail-to-Rail Micropower DACs in SO-8
• Manufacturer: LINER [Linear Technology]

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 V
is used for the digitally controlled 0mA to 16mA current.
R
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.
TYPICAL APPLICATIO S
S
is a sense resistor and the op amp modulates the
CS/LD
CLK
D
IN
LT
IN
®
1121-3.3
OUT
OPTO-ISOLATORS
500
ISOLATED
1 F
INPUTS
OPTO-
FROM
4N28
U
3.3V
10k
An Isolated 4mA to 20mA Process Controller
CLK
D
CS/LD
CLK
D
CS/LD
IN
IN
Has 3.3V Minimum Loop Voltage
V
CC
LTC1453
V
REF
OUT
V
OUT
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 D
guaranteed unless a clock edge with a rise time of less than
100ns is provided.
90k
45k
5k
5k
3k
–
+
LT1077
LTC1452/LTC1453
1k
11451/2/3 TA04
Q1
2N3440
R
10
OUT
S
V
3.3V TO 30V
I
OUT
LOOP
and D
LTC1451
IN
are not
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