OP295/OP495

Rev. G | Page 11 of 16

V+

–

+

–

+

5

3

2

1

6

8

4

7

1/2

OP295/

OP495

1/2

OP295/

OP495

+

–

VIN

VO

R4

100kΩ

R3

20kΩ

R2

20kΩ

R1

100kΩ

VREF

RG

VO =

(5+ 200kΩ) VIN+VREF

RG

Figure 24. Single-Supply Instrumentation Amplifier

Resistor RG sets the gain of the instrumentation amplifier.

Minimum gain is 6 (with no RG). All resistors should be matched

in absolute value as well as temperature coefficient to maximize

common-mode rejection performance and minimize drift. This

instrumentation amplifier can operate from a supply voltage as

low as 3 V.

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 kΩ and 2.55 MΩ, developing a voltage

that is servoed to 1.235 V, which is established by the AD589

band gap reference. The 3-wire RTD provides an equal line

resistance drop in both 100 Ω legs of the bridge, thus improving

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.

ZERO ADJ

AD589

37.4kΩ

5V

1.235

3

2

4

5

6

8

1

7

5V

AMP04

50Ω

332Ω

0.22µF

VO

–

+

2

3

1

+

–

200Ω

10-TURNS

26.7kΩ

0.5%

26.7kΩ

0.5%

100Ω

RTD

100Ω

0.5%

2.55MΩ

1%

6.19kΩ

1%

1/2

OP295/

OP495

4.5V = 450°C

0V = 0°C

Figure 25. Low Power RTD Amplifier

COLD JUNCTION COMPENSATED, BATTERY-

POWERED THERMOCOUPLE AMPLIFIER

The 150 μA quiescent current per amplifier consumption of the

OP295/OP495 makes them useful for battery-powered temperature

measuring instruments. The K-type thermocouple terminates

into an isothermal block where the terminated junctions’ ambient

temperatures can be continuously monitored and corrected by

summing an equal but opposite thermal EMF to the amplifier,

thereby canceling the error introduced by the cold junctions.

AD589

ALUMEL

CHROMEL

AL

CR

1N914

24.9kΩ

1.235V

9V

1.33MΩ

20kΩ

2

3

4

8

–

+

–

+

–

+

1

COLD

JUNCTIONS

ISOTHERMAL

BLOCK

K-TYPE

THERMOCOUPLE

40.7µV/°C

VO

5V = 500°C

0V = 0°C

OP295/

OP495

SCALE

ADJUST

7.15kΩ

1%

1.5MΩ

1%

24.9kΩ

1%

475Ω

1%

2.1kΩ

1%

24.3kΩ

1%

4.99kΩ

1%

500Ω

10-TURN

ZERO

ADJUST

Figure 26. Battery-Powered, Cold-Junction Compensated

Thermocouple Amplifier

To calibrate, immerse the thermocouple measuring junction in

a 0°C ice bath and adjust the 500 Ω zero-adjust potentiometer

to 0 V out. Then immerse the thermocouple in a 250°C tem-

perature bath or oven and adjust the scale-adjust potentiometer

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 achievable without linearization.

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

the rail-to-rail swing allows temperature measurements to 700°C.

However, linearization may be necessary for temperatures above

250°C, where the thermocouple becomes rather nonlinear. The

circuit draws just under 500 μA supply current from a 9 V

battery.

5 V ONLY, 12-BIT DAC THAT SWINGS 0 V TO 4.095 V

Figure 27 shows a complete voltage output DAC with wide

output voltage swing operating off a single 5 V supply. The

serial input, 12-bit DAC is configured as a voltage output device

with the 1.235 V reference feeding the current output pin (IOUT)

of the DAC. The VREF, which is normally the input, now becomes

the output.

The output voltage from the DAC is the binary weighted voltage

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

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