OP290_15 Datasheet, PDF (10/12 Pages) Analog Devices – Precision, Low Power, Micropower Dual Operational Amplifier

OP290
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.
I _OUT
=
V_TEMP (R6 + R7
R2 × R10
)
−
V_SET


R2 ×
R2
R6 × R7
× R10


(1)
The change in output current with temperature is the derivative
of the following transfer function:
∆I _OUT
=
∆V_TEMP
∆T
(R6 + R7)
(2)
∆T
R2 × R10
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
( ) I_OUT
=

∆I _FS
∆T_OPERATING
 ^TA
− T_MIN
+ 4 mA
Table 5 shows the values of R6 that are required for various
temperature ranges.
Table 5.
Temperature Range
0°C to +70°C
−40°C to +85°C
−50°C to +150°C
R6 (k Ω)
10
6.2
3
V_IN ²
V_OUT ⁶
REF43
V_TEMP
3
R1
GND ^{4 10kΩ}
2
8
1/2
OP290GP
1
3
4
V_TEMP
R2
1kΩ
R3
R5
100kΩ
5kΩ
R6
R4
3kΩ
20kΩ
SPAN TRIM
R7
5kΩ
6
1/2
OP290GP
V_{SET 5}
ZERO
TRIM
R8
7
1kΩ
R9
100kΩ
1N4002
V+
8V TO 40V
2N1711
R10
100Ω
1%, 1/2W
I_OUT
R_L
Figure 20. Temperature to 4 mA to 20 mA Transmitter
Rev. C | Page 10 of 12