Datasheet

9
LTC1658
An Optoisolated 4mA to 20mA Process Controller
CLK
D
IN

CS/LD
1µF
IN OUT
Q1
2N3440
R
S

10Ω
I
OUT
V
LOOP

3.8V TO 30V
V
CC
REF
V
OUT
FROM
OPTO-
ISOLATED
INPUTS
LTC1658
1658 TA04
LT
®
1121-3.3
3.01k
1%
–
+
+
LT1077
1k
20k
237k
1%
5k
60.4k
1%
OPTOISOLATORS
500Ω
3.3V
3.6k
4N28
CLK
D
IN

CS/LD
CLK
D
IN

CS/LD
This circuit shows how to use an LTC1658 to make an
optoisolated digitally controlled 4mA to 20mA process
controller. The controller circuitry, including the
optoisolation, is powered by the loop voltage that can have
a wide range of 3.8V to 30V. The 3.3V output of the
LT1121-3.3 is used for the 4mA offset current and V
OUT
is
used for the digitally controlled 0mA to 16mA current. R
S
is a sense resistor and the op amp modulates the transis-
tor 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
U