Datasheet

OP290
Rev. C | Page 10 of 12
APPLICATIONS INFORMATION
TEMPERATURE TO 4 MA TO 20 mA TRANSMITTER
A simple temperature to 4 mA to 20 mA transmitter is shown in
Figure 20. After calibration, the transmitter is accurate to +0.5°C
over the −50°C to +150°C temperature range. The transmitter
operates from 8 V to 40 V with supply rejection better than
3 ppm/V. One half of the OP290 is used to buffer the V
TEMP
pin
while the other half regulates the output current to satisfy the
current summation at its noninverting input.








×
××
−
×
+
=
R10R2
R7R6R2
V
R10R2
R7RV
I
SET
TEMP
OUT
)6(
(1)
The change in output current with temperature is the derivative
of the following transfer function:
R10R2
R7(R6
T
V
T
I
TEMP
OUT
×
+
∆
∆
=
∆
∆
)
(2)
From Equation 1 and Equation 2, it can be seen that if the span
trim is adjusted before the zero trim, the two trims are not
interactive, which greatly simplifies the calibration procedure.
Calibration of the transmitter is simple. First, the slope of the
output current vs. temperature is calibrated by adjusting the
span trim, R7. A couple of iterations may be required to ensure
that the slope is correct.
Once the span trim has been completed, the zero trim can be
made. Remember that adjusting the offset trim does not affect
the gain. The offset trim can be set at any known temperature
by adjusting R5 until the output current equals
( )
mA4+−








∆
∆
=
MIN
A
OPERATING
FS
OUT
TT
T
I
I
Table 5 shows the values of R6 that are required for various
temperature ranges.
Table 5.
Temperature Range R6 (k Ω)
0°C to +70°C 10
−40°C to +85°C 6.2
−50°C to +150°C 3
1/2
OP290GP
V
IN
V
OUT
V
TEMP
GND
6
3
2
4
10kΩ
R1
REF43
2N1711
V+
8V TO 40V
1N4002
SPAN TRIM
R10
100Ω
1%, 1/2W
R9
100kΩ
R8
1kΩ
R3
100kΩ
R5
5kΩ
R2
1kΩ
V
TEMP
4
8
2
3
ZERO
TRIM
V
SET
R4
20kΩ
R6
3kΩ
R7
5kΩ
I
OUT
R
L
7
6
5
1
1/2
OP290GP
00327-021
Figure 20. Temperature to 4 mA to 20 mA Transmitter