AMP04

REV. A

–10–

APPLICATION CIRCUITS

Low Power Precision Single Supply RTD Amplifier

Figure 11 shows a linearized RTD amplifier that is powered off

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 cur-

rent is divided down from the 2.500 volt reference. The AMP04

amplifies the bridge output to a 10 mV/

°C output coefficient.

100

Ω

7

1

6

5

4

2

3

8

+5V

AMP-04

8

7

4

5

6

+5V

50k

R7

121k

1/2

OP-295

1/2

OP-295

A

B

REF-43

GND

OUT

IN

6

4

2

23

1

+5V

C2

0.1

µF

R1

26.7k

R2

26.7k

R4

100

Ω

RTD

100

Ω

R

SENSE

1k

R9

50

Ω

R8

383

Ω

C3

0.1

µF

C1

0.47

µF

R10

R6

11.5k

2.5V

R5

1.02k

500

Ω

0.202V

V

OUT

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

R3

BALANCE

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 tem-

perature increases. When calibrated properly, the RTD’s non-

linearity 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. There-

fore, be sure to adjust R3 to first cause the output to swing

positive and then back off until the output just stop swinging

negatively.

Next, set the LINEARITY ADJ. potentiometer to the mid-

range. 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 con-

verted to a current by the output amplifier. The total quiescent

current drawn by the circuit, which includes the bridge, the am-

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

null current that flows through the current-sense resistor

put, 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 in-

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

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

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

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

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