OP191GS datasheet(16/20 Pages) AD

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OP191/OP291/OP491

REV. 0

–16–

A +3 V, Cold Junction Compensated Thermocouple Amplifier

The OP291’s low supply operation makes it ideal for +3 V

battery powered applications such as the thermocouple amplifier

shown in Figure 64. The K-type thermocouple terminates in an

isothermal block where the junctions’ ambient temperature is

continuously monitored using a simple 1N914 diode. The

diode corrects the thermal EMF generated in the junctions by

feeding a small voltage, scaled by the 1.5 M

Ω and 475 Ω

resistors, to the op amp.

To calibrate this circuit, immerse the thermocouple measuring

junction in a 0

°C ice bath, and adjust the 500 Ω pot to zero

volts out. Next, immerse the thermocouple in a 250

°C tem-

perature bath or oven and adjust the Scale Adjust pot for an

output voltage of 2.50 V. Within this temperature range, the

K-type thermocouple is accurate to within

±3°C without

linearization.

OP291

4.99k

500

Ω

10-TURN

ZERO

ADJUST

24.3k

7.15k

24.9k

2.1k

475

Ω

1.5M

1N914

ISOTHERMAL

BLOCK

ALUMEL

CHROMEL

COLD

JUNCTIONS

K-TYPE

THERMOCOUPLE

40.7

µV/°C

10k

3.0V

AD589

1.33M

Ω 20k

SCALE

ADJUST

OUT

0V = 0

°C

3V = 300

°C

1.235V

11.2mV

Figure 64. A 3 V, Cold Junction Compensated Thermo-

couple Amplifier

Single Supply, Direct Access Arrangement for Modems

An important building block in modems is the telephone line

interface. In the circuit shown in Figure 65, a direct access

arrangement is utilized for transmitting and receiving data from

the telephone line. Amplifier A1 is the receiving amplifier, and

amplifiers A2 and A3 are the transmitters. The forth amplifier,

A4, generates a pseudo ground half way between the supply

voltage and ground. This pseudo ground is needed for the ac

coupled bipolar input signals.

Figure 65. Single Supply Direct Access Arrangement for

Modems

The transmit signal, TXA, is inverted by A2 and then re-

inverted by A3 to provide a differential drive to the transformer,

where each amplifier supplies half the drive signal. This is

needed because of the smaller swings associated with a single

supply as opposed to a dual supply. Amplifier A1 provides

some gain for the received signal, and it also removes the

transmit signal present at the transformer from the receive

signal. To do this, the drive signal from A2 is also fed to the

noninverting input of A1 to cancel the transmit signal from the

transformer. The OP491’s bandwidth of 3 MHz and rail-to-rail

output swings ensures that it can provide the largest possible

drive to the transformer at the frequency of transmission.

RXA

1/4

OP491

1/4

OP491

1/4

OP491

37.4k

Ω

3.3k

Ω

0.0047µF

20k

Ω, 1%

475

Ω, 1%

0.033µF

37.4k

Ω, 1%

390pF

750pF

0.1µF

1:1

5.1V TO 6.2V

ZENER 5

1/4

OP491

100k

+3V OR +5V

10µF

0.1µF

20k

Ω, 1%

20k

Ω, 1%

20k

Ω 1%

TXA

20k

Ω 1%

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