LTC1655I Linear Technology, LTC1655I Datasheet - Page 8

• Manufacturer Part Number: LTC1655I
• Description: 16-Bit Rail-to-Rail Micropower DAC in SO-8 Package
• Manufacturer: Linear Technology

This circuit shows how to use an LTC1655 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 6V to 30V. The 2.048V reference output of
the LTC1655 is used for the 4mA offset current and V
LTC1655
TYPICAL APPLICATIONS
8
CS/LD
CLK
D
IN
IN
LT
®
1121-5
OUT
OPTOISOLATORS
500
OPTOISOLATED
1 F
INPUTS
FROM
4N28
U
5V
10k
An Isolated 4mA to 20mA Process Controller
CLK
D
CS/LD
IN
CLK
D
CS/LD
IN
V
CC
LTC1655
V
REF
OUT
V
OUT
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.
150k
1%
75k
1%
S
is a sense resistor and the op amp modulates the
20k
5k
3k
+
–
LT
®
1077
1k
Q1
2N3440
R
10
1655 TA03
S
V
6V TO 30V
I
OUT
LOOP