LTC1655LCS8 Datasheet, PDF (11/16 Pages) Linear Technology – 16-Bit Rail-to-Rail Micropower DACs in

LTC1655/LTC1655L
TYPICAL APPLICATIONS
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_OUT
is used for the digitally controlled 0mA to 16mA current.
R_S 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.
An Isolated 4mA to 20mA Process Controller
LT^®1121-5
IN OUT
1µF
FROM
OPTOISOLATED
INPUTS
150k
1% 20k
8
6
1
V_CC
CLK
V_REF
75k
2
D_IN
LTC1655 VOUT ^{7 1%}
5k
3 _CS/LD
GND
5
3₊
7
LT^®1077
3k ² _–
4
6 ^1k
CLK
D_IN
CS/LD
5V
OPTOISOLATORS
500Ω
4N28
10k
CLK
D_IN
CS/LD
V_LOOP
6V TO 30V
Q1
2N3440
R_S
10Ω
I_OUT
1655/55L TA03
11