OP295/OP495

REV. B

–9–

common-mode rejection performance and minimize drift. This

instrumentation amplifier can operate from a supply voltage as

low as 3 volts.

A Single Supply RTD Thermometer Amplifier

This RTD amplifier takes advantage of the rail-to-rail swing of

the OP295/OP495 to achieve a high bridge voltage in spite of a

low 5 V supply. The OP295/OP495 amplifier servos a constant

200

µA current to the bridge. The return current drops across

the parallel resistors 6.19 k

Ω and the 2.55 MΩ, developing a

voltage that is servoed to 1.235 V, which is established by the

AD589 bandgap reference. The 3-wire RTD provides an equal

line resistance drop in both 100

Ω legs of the bridge, thus im-

proving the accuracy.

The AMP04 amplifies the differential bridge signal and converts

it to a single-ended output. The gain is set by the series resis-

tance of the 332

Ω resistor plus the 50 Ω potentiometer. The

gain scales the output to produce a 4.5 V full scale. The

0.22

µF capacitor to the output provides a 7 Hz low-pass filter

to keep noise at a minimum.

3

7

1

8

6

5

4

2

AMP04

0.22

µF

332

Ω

50

Ω

26.7k

0.5%

100

Ω

0.5%

100

Ω

RTD

2.55M

1%

6.19k

1%

AD589

1.235

37.4k

+5V

200

Ω

10-TURNS

ZERO ADJ

26.7k

0.5%

V

O

4.5V = 450

°C

0V = 0

°C

+5V

1

2

3

1/2

OP295/

OP495

Figure 6. Low Power RTD Amplifier

A Cold Junction Compensated, Battery Powered

Thermocouple Amplifier

The OP295/OP495’s 150

µA quiescent current per amplifier

consumption makes it useful for battery powered temperature

measuring instruments. The K-type thermocouple terminates

into an isothermal block where the terminated junctions’ ambi-

ent temperatures can be continuously monitored and corrected

by summing an equal but opposite thermal EMF to the ampli-

fier, thereby canceling the error introduced by the cold junctions.

V

O

0V = 0

°C

5V = 500

°C

4.99k

1%

1.33M

Ω 20k

SCALE

ADJUST

9V

24.3k

1%

24.9k

7.15k

1%

1.235V

AD589

24.9k

1%

500

Ω

10-TURN

2.1k

1%

475

Ω

1%

1.5M

1%

1N914

ISOTHERMAL

BLOCK

COLD

JUNCTIONS

CR

AL

CHROMEL

K-TYPE

THERMOCOUPLE

40.7

µV/°C

ZERO

ADJUST

8

1

4

2

3

ALUMEL

OP295/

OP495

Figure 7. Battery Powered, Cold-Junction Compensated

Thermocouple Amplifier

To calibrate, immerse the thermocouple measuring junction in a

0

°C ice bath, adjust the 500 Ω Zero Adjust pot to zero volts out.

Then immerse the thermocouple in a 250

°C temperature bath

or oven and adjust the Scale Adjust pot for an output voltage of

2.50 V, which is equivalent to 250

°C. Within this temperature

range, the K-type thermocouple is quite accurate and produces

a fairly linear transfer characteristic. Accuracy of

±3°C is achiev-

able without linearization.

Even if the battery voltage is allowed to decay to as low as 7 volts,

the rail-to-rail swing allows temperature measurements to

700

°C. However, linearization may be necessary for tempera-

tures above 250

°C where the thermocouple becomes rather

nonlinear. The circuit draws just under 500

µA supply current

from a 9 V battery.

A 5 V Only, 12-Bit DAC That Swings 0 V to 4.095 V

Figure 8 shows a complete voltage output DAC with wide out-

put voltage swing operating off a single +5 V supply. The serial

input 12-bit D/A converter is configured as a voltage output

device with the 1.235 V reference feeding the current output pin

becomes the output.

The output voltage from the DAC is the binary weighted volt-

age of the reference, which is gained up by the output amplifier

such that the DAC has a 1 mV per bit transfer function.

+5V

R2

41.2k

R3

5k

Ω

R4

100k

Ω

V

DD

R

FB

V

REF

SRI

CLK

GND

I

OUT

DAC8043

3

4

7

65

1

2

8

+5V

DIGITAL

CONTROL

+5V

AD589

R1

17.8k

Ω

+1.23V

LD

V

(4.096V)

D

4096

TOTAL POWER DISSIPATION = 1.6mW

8

1

4

2

3

OP295/

OP495

Figure 8. A 5 Volt 12-Bit DAC with 0 V to +4.095 Output

Swing

4–20 mA Current Loop Transmitter

Figure 9 shows a self powered 4–20 mA current loop transmit-

ter. The entire circuit floats up from the single supply (12 V to

36 V) return. The supply current carries the signal within the 4

to 20 mA range. Thus the 4 mA establishes the baseline

8

1

2

3

220pF

220

Ω

REF02

GND

62

4

100

Ω

2N1711

100k

1%

HP

5082-2800

100

Ω

1%

V

IN

0 + 3V

100k

Ω

10-TURN

1.21M

1%

NULL ADJ

SPAN ADJ

182k

1%

10k

Ω

10-TURN

5V

+12V

TO

+36V

R

L

100

Ω

4–20mA

4

1/2

OP295/

OP495

Figure 9. 4–20 mA Current Loop Transmitter