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參數(shù)資料
| 型號： | 522948H |
| 英文描述： | IC |
| 中文描述： | 集成電路 |
| 文件頁數(shù)： | 9/12頁 |
| 文件大?。?/td> | 174K |
| 代理商： | 522948H |
9
LTC1451
LTC1452/LTC1453
An Isolated 4mA to 20mA Process Controller
Has 3.3V Minimum Loop Voltage
11451/2/3 TA04
3k
10k
1k
45k
5k
90k
5k
Q1
2N3440
R
S
10
V
3.3V TO 30V
I
OUT
OUT
IN
CLK
D
IN
CS/LD
CLK
D
IN
CS/LD
CLK
D
IN
CS/LD
V
CC
V
OUT
1
μ
F
LTC1453
4N28
OPTO-ISOLATORS
3.3V
500
LT
1121-3.3
FROM
OPTO-
ISOLATED
INPUTS
V
REF
+
LT1077
–
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
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.
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
and D
IN
are not
guaranteed unless a clock edge with a rise time of less than
100ns is provided.
TYPICAL APPLICATIO S
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