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 VTEMP pin

while the other half regulates the output current to satisfy the

current summation at its noninverting input.

















×

×

×

−

×

+

=

R10

R2

R7

R6

R2

V

R10

R2

R7

R

V

I

SET

TEMP

OUT

)

6

(

(1)

The change in output current with temperature is the derivative

of the following transfer function:

R10

R2

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

(

) mA

4

+

−













∆

∆

=

MIN

A

OPERATING

FS

OUT

T

T

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

VIN

VOUT

VTEMP

GND

6

3

2

4

10k

Ω

R1

REF43

2N1711

V+

8V TO 40V

1N4002

SPAN TRIM

R10

100

Ω

1%, 1/2W

R9

100k

Ω

R8

1k

Ω

R3

100k

Ω

R5

5k

Ω

R2

1k

Ω

VTEMP

4

8

2

3

ZERO

TRIM

VSET

R4

20k

Ω

R6

3k

Ω

R7

5k

Ω

IOUT

RL

7

6

5

1

1/2

OP290GP

Figure 20. Temperature to 4 mA to 20 mA Transmitter