6

LT1025

1025fb

Figure 4. Thermocouple Nonlinearity, 0

°C to 400°C

Figure 5. Offset Curve Fitting

In many situations, thermocouples are used in high noise

environments, and some sort of input filter is required.

(See discussion of input filters). To reject 60Hz pick-up

with reasonable capacitor values, input resistors in the

10k-100k range are needed. Under these conditions, bias

current for the amplifier needs to be less than 1nA to avoid

offset and drift effects.

To avoid gain error, high open loop gain is necessary for

single-stagethermocoupleamplifierswith10mV/

°Corhigher

outputs. A type K amplifier, for instance, with 100mV/

°C

output, needs a

closed loop gain of

≈2,500. An ordinary op

amp with a minimum open loop of 50,000 would have an

initial gain error of (2,500)/(50,000) = 5%! Although closed

loop gain is commonly trimmed, temperature drift of open

loop gain will have a very deleterious effect on output

accuracy. Minimum suggested open loop gain for type E, J,

K, and T thermocouples is 250,000. This gain is adequate for

type R and S if output scaling is 10mV/

°C or less.

Suggested Amplifier Types

SUPPLY VOLTAGE

THERMOCOUPLE

±15V

±5V

SINGLE SUPPLY

E, J, K, T

LTKA0x

LTKA0x

LTC1050

LT1012

LT1012

LTC1052

LT1001

LT1001

LT1006

LTC1050

LTC1052

LT1006

R, S

LTKA0x

LTC1050

LTC1050

LT1012

LTC1052

LTC1052

LTKA0x

LT1006

Thermocouple Nonlinearities

Thermocouples are linear over relatively limited temperature

spans if accuracies of better than 2

°C are needed. The graph

in Figure 4 shows thermocouple nonlinearity for the

temperature range of 0

°C to 400°C. Nonlinearities can be

dealtwithinhardwarebyusingoffsets,breakpoints,orpower

series generators. Software solutions include look-up tables,

power series expansions, and piece-wise approximations.

For tables and power series coefficients, the reader is referred

to the ASTM Publication 470A.

Hardware correction for nonlinearity can be as simple as an

offset term. This is shown in Figure 5. The thermocouple

shown in the figure has an increasing slope (

α) with

calibrating at T1/6 or T5/6 can significantly reduce errors. The

technique is as follows:

1. Calculate amplifier gain:

SF = Output scale factor, e.g., 10mV/

°C

2. Use precision resistors to set gain or calibrate gain by

introducing a precision “delta” input voltage and trimming

for proper “delta” output.

APPLICATIO S I FOR ATIO

TEMPERATURE (

°C)

0

20

15

12.5

10

5

100

200

250

LT1025 • G04

17.5

2.5

0

7.5

8

6

5

4

2

7

1

0

3

50

150

300 350

400

K

→SCALE

J

→SCALE

SCALE

←E

SCALE

←T

TEMPERATURE (

°C)

0

LT1025 • G05

T1/6

T5/6

THERMOCOUPLE

SIMPLE AMPLIFIER

OFFSET AMPLIFIER

ERROR AFTER OFFSETTING

ERROR BEFORE OFFSETTING