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AD8494ARMZ

Precision Thermocouple Amplifiers with Cold Junction Compensation

Analog Devices 

THEORY OF OPERATION

THERMOCOUPLES

A thermocouple is a rugged, low cost temperature transducer

whose output is proportional to the temperature difference

between a measurement junction and a reference junction. It

has a very wide temperature range. Its low level output (typically

tens of microvolts per °C) requires amplification. Variation in

the reference junction temperature results in measurement

error unless the thermocouple signal is properly compensated.

A thermocouple consists of two dissimilar metals. These metals

are connected at one end to form the measurement junction,

also called the hot junction. The other end of the thermocouple

is connected to the metal lines that lead to the measurement

electronics. This connection forms a second junction: the

reference junction, also called the cold junction.

MEASUREMENT

JUNCTION

REFERENCE

JUNCTION

PCB

TRACES

AD849x

THERMOCOUPLE WIRES

Figure 26. Thermocouple Junctions

To derive the temperature at the measurement junction (TMJ),

the user must know the differential voltage created by the thermo-

couple. The user must also know the error voltage generated by

the temperature at the reference junction (TRJ). Compensating

for the reference junction error voltage is typically called cold

junction compensation. The electronics must compensate for

any changes in temperature at the reference (cold) junction so

that the output voltage is an accurate representation of the hot

junction measurement.

THERMOCOUPLE SIGNAL CONDITIONER

The AD8494/AD8495/AD8496/AD8497 thermocouple amplifiers

provide a simple, low cost solution for measuring thermocouple

temperatures. These amplifiers simplify many of the difficulties

of measuring thermocouples. An integrated temperature sensor

performs cold junction compensation. A fixed-gain instrumen-

tation amplifier amplifies the small thermocouple voltage to

provide a 5 mV/°C output. The high common-mode rejection

of the amplifier blocks common-mode noise that the long

thermocouple leads can pick up. For additional protection, the

high impedance inputs of the amplifier make it easy to add

extra filtering.

Table 6 shows an example of a J type thermocouple voltage for

various combinations of 0°C and 50°C on the reference and

measurement junctions. Table 6 also shows the performance

of the AD8494 amplifying the thermocouple voltage and

compensating for the reference junction temperature changes,

thus eliminating the error.

AD8494/AD8495/AD8496/AD8497

Table 6. J Type Thermocouple Voltages and AD8494 Readings

Measurement

Junction

Temperature

(TMJ)

Reference

Junction

Temperature

(TRJ)

Thermocouple

Voltage

AD8494

Reading

50°C

0°C

+2.585 mV

250 mV

50°C

50°C

0 mV

250 mV

0°C

0°C

0 mV

0 mV

0°C

50°C

−2.585 mV

0 mV

AD8494/AD8495/AD8496/AD8497 ARCHITECTURE

Figure 27 shows a block diagram of the AD849x circuitry. The

AD849x consists of a low offset, fixed-gain instrumentation

amplifier and a temperature sensor.

SENSE

–IN

1MΩ

ESD AND

OVP

AD8494/AD8495/

AD8496/AD8497

A2

THERMO-

COUPLE

COLD JUNCTION

COMPENSATION

A3

OUT

+IN

ESD AND

A1

OVP

REF

Figure 27. Block Diagram

The AD849x output is a voltage that is proportional to the tem-

perature at the measurement junction of the thermocouple (TMJ).

To derive the measured temperature from the AD849x output

voltage, use the following transfer function:

TMJ = (VOUT − VREF)/(5 mV/°C)

An ideal AD849x achieves this output with an error of less than

±2°C, within the specified operating ranges listed in Table 7.

Instrumentation Amplifier

A thermocouple signal is so small that considerable gain is

required before it can be sampled properly by most ADCs. The

AD849x has an instrumentation amplifier with a fixed gain that

generates an output voltage of 5 mV/°C for J type and K type

thermocouples.

VOUT = (TMJ × 5 mV/°C) + VREF

To accommodate the nonlinear behavior of the thermocouple,

each amplifier has a different gain so that the 5 mV/°C is accu-

rately maintained for a given temperature measurement range.

• The AD8494 and AD8496 (J type) have an instrumentation

amplifier with a gain of 96.7 and 90.35, respectively.

• The AD8495 and AD8497 (K type) have an instrumentation

amplifier with a gain of 122.4.

Rev. 0 | Page 11 of 16

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