ltc1658is8-trpbf Linear Technology Corporation, ltc1658is8-trpbf Datasheet - Page 9

• Manufacturer Part Number: ltc1658is8-trpbf
• Description: Ltc1658 - 14-bit Rail-to-rail Micropower Dac
• Manufacturer: Linear Technology Corporation

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
TYPICAL APPLICATIO S
CS/LD
CLK
D
IN
IN
LT
®
1121-3.3
OUT
ISOLATED
OPTOISOLATORS
INPUTS
500
OPTO-
FROM
+
1 F
U
4N28
CLK
D
CS/LD
An Optoisolated 4mA to 20mA Process Controller
IN
V
3.3V
CC
LTC1658
3.6k
REF
CLK
D
CS/LD
IN
V
OUT
OUT
is
60.4k
237k
1%
1%
used for the digitally controlled 0mA to 16mA current. R
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.
20k
5k
3.01k
1%
1658 TA04
+
–
LT1077
1k
Q1
2N3440
R
10
S
V
3.8V TO 30V
I
OUT
LOOP
LTC1658
9
S