LTC1655/LTC1655L
U
TYPICAL APPLICATIONS
This circuit shows how to use an LTC1655 to make an is used for the digitally controlled 0mA to 16mA current.
optoisolated digitally controlled 4mA to 20mA process RS is a sense resistor and the op amp modulates the
controller. The controller circuitry, including the transistor Q1 to provide the 4mA to 20mA current through
optoisolation, ispoweredbytheloopvoltagethatcanhave this resistor. The potentiometers allow for offset and full-
a wide range of 6V to 30V. The 2.048V reference output of scale adjustment. The control circuitry dissipates well
the LTC1655 is used for the 4mA offset current and VOUT under the 4mA budget at zero scale.
An Isolated 4mA to 20mA Process Controller
V
LOOP
6V TO 30V
150k
1%
LT®1121-5
IN OUT
20k
5k
8
6
1µF
V
V
REF
CC
1
2
3
75k
1%
CLK
FROM
7
3
2
7
V
LTC1655
D
+
OUT
OPTOISOLATED
INPUTS
IN
1k
6
Q1
2N3440
LT®1077
CS/LD
GND
3k
–
4
5
R
S
5V
10Ω
OPTOISOLATORS
I
OUT
10k
CLK
IN
1655/55L TA03
D
CS/LD
CLK
IN
CS/LD
500Ω
4N28
D
11
Linear [ Linear ]
