AMP04

REV. B

–10–

APPLICATION CIRCUITS

Low Power Precision Single Supply RTD Amplifier

Figure 11 shows a linearized RTD amplifier that is powered

from a single 5 volt supply. However, the circuit will work up to

36 volts without modification. The RTD is excited by a 100

µA

constant current that is regulated by amplifier A (OP295). The

0.202 volts reference voltage used to generate the constant current

is divided down from the 2.500 volt reference. The AMP04 ampli-

fies the bridge output to a 10 mV/

°C output coefficient.

R10

100

R2

26.7k

1k

FULL-SCALE

ADJ

0

4.00V

(0 C TO 400 C)

LINEARITY

ADJ.

(@1/2 FS)

NOTES: ALL RESISTORS

0.5%,

25 PPM/ C

ALL POTENTIOMETERS

25 PPM/ C

AMP04

R8

383

R9

50

C1

0.47 F

1

7

3

2

4

5

8

6

1/2

OP295

1/2

OP295

R1

26.7k

500

R4

100

R6

11.5k

R5

1.02k

OUT

REF43

IN

GND

2.5V

R7

121k

50k

4

5

6

8

7

5V

2

5V

C2

0.1 F

C3

0.1 F

RTD

100

0.202V

6

4

1

2

3

R3

BALANCE

5V

A

B

Figure 11. Precision Single Supply RTD Thermometer

Amplifier

The RTD is linearized by feeding a portion of the signal back to

the reference circuit, increasing the reference voltage as the

temperature increases. When calibrated properly, the RTD’s

nonlinearity error will be canceled.

To calibrate, either immerse the RTD into a zero-degree ice

bath or substitute an exact 100

Ω resistor in place of the RTD.

Then adjust bridge BALANCE potentiometer R3 for a 0 volt

output. Note that a 0 volt output is also the negative output swing

limit of the AMP04 powered with a single supply. Therefore, be

sure to adjust R3 to first cause the output to swing positive and

then back off until the output just stops swinging negatively.

Next, set the LINEARITY ADJ potentiometer to the midrange.

Substitute an exact 247.04

Ω resistor (equivalent to 400°C

temperature) in place of the RTD. Adjust the FULL-SCALE

potentiometer for a 4.000 volts output.

Finally substitute a 175.84

Ω resistor (equivalent to 200°C

temperature), and adjust the LINEARITY ADJ potentiometer

for a 2.000 volts at the output. Repeat the full-scale and the

half-scale adjustments as needed.

When properly calibrated, the circuit achieves better than

±0.5°C accuracy within a temperature measurement range

from 0

°C to 400°C.

Precision 4-20 mA Loop Transmitter with Noninteractive Trim

Figure 12 shows a full bridge strain gage transducer amplifier

circuit that is powered off the 4-20 mA current loop. The AMP04

amplifies the bridge signal differentially and is converted to a

current by the output amplifier. The total quiescent current

drawn by the circuit, which includes the bridge, the amplifiers,

and the resistor biasing, is only a fraction of the 4 mA null

whose common-mode is fixed at the current summing reference

voltage, thus regulating the output current.

With no bridge signal, the 4 mA null is simply set up by the

50 k

Ω NULL potentiometer plus the 976 kΩ resistors that

50 mV full-scale bridge voltage, the AMP04 amplifies the

voltage-to-current converter for a full-scale of 20 mA at the

output. Since the OP90’s input operates at a constant 0 volt

common-mode voltage, the null and the span adjustments do

not interact with one another. Calibration is simple and easy

with the NULL adjusted first, followed by SPAN adjust. The

entire circuit can be remotely placed, and powered from the

4-20 mA 2-wire loop.

20

U1

AMP04

5k

10-TURN

2.49k

1

7

3

2

4

5

8

6

U2

OP90

50mV

FS

0.22 F

97.6k

3

2

B

976k

50k

HP

5082-2810

220pF

4

6

100k

5%

2k

5%

T1P29A

0.1 F

5.00V

OUT

REF02

N

GND

U3

1N4002

4mA NULL

13.3k

15.8k

3500

STRAIN

GAGE BRIDGE

7

20mA

SPAN

6

2

4

12V TO 36V

100

4-20mA

UNLESS OTHERWISE SPECIFIED, ALL RESISTORS

1%

OR BETTER POTENTIOMETER < 50 PPM/ C

Figure 12. Precision 4-20 mA Loop Transmitter Features Noninteractive Trims