OP191GSZ datasheet(21/24 Pages) AD

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OP191/OP291/OP491

Rev. D | Page 21 of 24

A 3 V, COLD JUNCTION COMPENSATED

THERMOCOUPLE AMPLIFIER

The OP291 low supply operation makes it ideal for 3 V battery-

powered applications such as the thermocouple amplifier

shown in Figure 70. The K-type thermocouple terminates in an

isothermal block where the junction 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 MΩ 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 potentiometer

to 0 V out. Next, immerse the thermocouple in a 250°C

temperature bath or oven and adjust the scale adjust

potentiometer 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

500Ω

10 TURN

ZERO

ADJUST

24.3kΩ

7.15kΩ

24.9kΩ

2.1kΩ

475Ω

1.5MΩ

1N914

ISOTHERMAL

BLOCK

ALUMEL

CHROMEL

COLD

JUNCTIONS

K-TYPE

THERMOCOUPLE

40.7μV/°C

10kΩ

3.0V

AD589

1.33MΩ 20kΩ

SCALE

ADJUST

VOUT

0V = 0°C

3V = 300°C

1.235V

11.2mV

4.99kΩ

1/2

Figure 70. A 3 V, Cold Junction Compensated Thermocouple 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 71, a direct access

arrangement is used to transmit and receive data from the

telephone line. Amplifier A1 is the receiving amplifier;

Amplifier A2 and Amplifier A3 are the transmitters. The fourth

amplifier, A4, generates a pseudo ground halfway between the

supply voltage and ground. This pseudo ground is needed for

the ac-coupled bipolar input signals.

The transmit signal, TXA, is inverted by A2 and then reinverted

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 received 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.

RXA

1/4

OP491

37.4kΩ

3.3kΩ

0.0047μF

20kΩ,1%

475Ω,1%

0.033μF

37.4kΩ,1%

390pF

750pF

0.1μF

1:1

5.1V TO 6.2V

ZENER 5

100kΩ

3V OR 5V

10μF0.1μF

20kΩ,1%

TXA

20kΩ,1%

1/4

OP491

1/4

OP491

1/4

OP491

Figure 71. Single-Supply, Direct Access Arrangement for Modems

The OP491 bandwidth of 3 MHz and rail-to-rail output swings

ensure that it can provide the largest possible drive to the

transformer at the frequency of transmission.

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