Pentek 6210 Operating Manual page 161

TYPICAL APPLICATIO S
®
LT 1121-3.3
IN OUT
OPTO-ISOLATORS
CLK
500Ω
D
IN
CS/LD
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 is a sense resistor and the op amp modulates the
S
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
Has 3.3V Minimum Loop Voltage
1µF
V V
CC REF
CLK
FROM
OPTO-
LTC1453
D
IN
ISOLATED
INPUTS
CS/LD
3.3V
10k
CLK
D
IN
CS/LD
4N28
90k 5k
45k 5k
V
OUT
3k
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
OUT
guaranteed unless a clock edge with a rise time of less than
100ns is provided.
LTC1451
LTC1452/LTC1453
V
LOOP
3.3V TO 30V
+
1k
Q1
LT1077
2N3440
–
R
S
10Ω
I
OUT
11451/2/3 TA04
and D
OUT
are not
IN
9