LTC2630
OPERATION
Optoisolated 4mA to 20mA Process Controller
additional current through Q1. Note that at the maximum
loop voltage of 80V, Q1 will dissipate 1.6W when I
20mA and must have an appropriate heat sink.
=
OUT
Figure 5 shows how to use an LTC2630HZ to make an
optoisolated, digitally-controlled 4mA to 20mA transmit-
ter. The transmitter circuitry, including optoisolation, is
powered by the loop voltage which has a wide range of
5.4V to 80V. The 5V output of the LT®3010-5 is used to
R
and R
are the closest 0.1% values to ideal
OFFSET
GAIN
for controlling a 4mA to 20mA output as the digital input
varies from zero scale to full scale. Alternatively, R
OFFSET
can be a 365k, 1% resistor in series with a 20k trim pot
set the 4mA offset current and V
is used to digitally
OUT
and R
can be a 75.0k, 1% resistor in series with a 5k
control the 0mA to 16mA signal current. The supply cur-
GAIN
trim pot. The optoisolators shown will limit the speed of
the serial bus; the 6N139 is an alternative that will allow
higher data rates.
rent for the regulator, DAC, and op amp is well below
the 4mA budget at zero scale. R senses the total loop
S
current, which includes the quiescent supply current and
CS/LD
SCK
SDI
1
2
3
4
5
6
7
8
9
10
11
12
D8
13
D7
14
D6
15
D5
16
D4
17
D3
18
D2
19
D1
20
D0
21
22
23
24
C3
C2
C1
C0
X
X
X
X
D11 D10 D9
X
X
X
X
2630 F03a
COMMAND WORD
4 DON’T-CARE BITS
DATA WORD
24-BIT INPUT WORD
Figure 3a. 24-Bit Load Sequence (Minimum Input Word)
LTC2630-12 SDI Data Word: 12-Bit Input Code + 4 Don’t-Care Bits (Shown);
LTC2630-10 SDI Data Word: 10-Bit Input Code + 6 Don’t-Care Bits;
LTC2630-8 SDI Data Word: 8-Bit Input Code + 8 Don’t-Care Bits
CS/LD
SCK
1
2
3
4
5
6
7
8
9
10
C2
11
C1
12
C0
13
14
15
16
17
18
19
20
D8
21
D7
22
D6
23
D5
24
D4
25
D3
26
D2
27
D1
28
D0
29
30
31
32
SDI
X
X
X
X
X
X
X
X
C3
X
X
X
X
D11 D10 D9
X
X
X
X
8 DON’T-CARE BITS
COMMAND WORD
4 DON’T-CARE BITS
DATA WORD
2630 F03b
32-BIT INPUT WORD
Figure 3b. 32-Bit Load Sequence
LTC2630-12 SDI Data Word: 12-Bit Input Code + 4 Don’t-Care Bits (Shown);
LTC2630-10 SDI Data Word: 10-Bit Input Code + 6 Don’t-Care Bits;
LTC2630-8 SDI Data Word: 8-Bit Input Code + 8 Don’t-Care Bits
2630f
16
Linear [ Linear ]
