4

LT1025

1025fb

an extender made of matching thermocouple wire can be

used. This shifts the cold junction from the user termina-

tion to the end of the extender so that the LT1025 can be

located remotely from the user termination as shown in

Figure 2.

The four thermocouple outputs on the LT1025 are

60.9

µV/°C (E), 51.7µV/°C (J), 40.6µV/°C (K and T), and

6

µV/°C (R and S). These particular coefficients are chosen

to match the room temperature (25

°C) slope of the

thermocouples. Over wide temperature ranges, however,

the slope of thermocouples changes, yielding a quasi-

parabolic error compared to a constant slope. The LT1025

outputs have a deliberate parabolic “bow” to help

compensate for this effect. The outputs can be mathemati-

cally described as the sum of a linear term equal to room

temperature slope plus a quadratic term proportional to

temperature deviation from 25

°C squared. The coefficient

(ß) of the quadratic term is a compromise value chosen to

offer improvement in all the outputs.

ß

The actual ß term which would be required to best

compensate each thermocouple type in the temperature

range of 0

The temperature error specification for the LT1025

10mV/

°C output (shown as a graph) assumes a ß of

if its 10mV/

Operating at Negative Temperatures

The LT1025 is designed to operate with a single positive

supply. It therefore cannot deliver proper outputs for

temperatures below zero unless an external pull-down

connected to any convenient negative supply. It should be

selected to sink at least 30

µA of current. Suggested value

for a –5V supply is 150k

Ω, and for a –15V supply, 470kΩ.

Smaller resistors must be used if an external load is

connected to the 10mV/

°C output. The LT1025 can source

up to 1mA of current, but there is a trade-off with internal

temperature rise.

Internal Temperature Rise

The LT1025 is specified for temperature accuracy assum-

ing no internal temperature rise. At low supply voltages

this rise is usually negligible (

≈ 0.05°C at 5V), but at higher

supply voltages or with external loads or pull-down cur-

rent, internal rise could become significant. This effect can

be calculated from a simple thermal formula,

to ambient, (

load current including actual load to ground and any pull-

down current needed to generate negative outputs. A

sample calculation with a 15V supply and 50

µA pull-down

current would yield, (130

°C/W) (15V) (80µA + 50µA) =

0.32

°C. This is a significant rise in some applications. It

can be reduced by lowering supply voltage (a simple fix is

calibrated and specified after an initial warm-up period of

several minutes.

Driving External Capacitance

The direct thermocouple drive pins on the LT1025 (J, K,

etc.) can be loaded with as much capacitance as desired,

but the 10mV/

°C output should not be loaded with more

than 50pF unless external pull-down current is added, or

a compensation network is used.

Thermocouple Effects in Leads

Thermocouple voltages are generated whenever dissimi-

lar materials are joined. This includes the leads of IC

packages, which may be kovar in TO-5 cans, alloy 42 or

copper in dual-in-line packages, and a variety of other

materials in plating finishes and solders. The net effect

of these thermocouples is “zero” if all are at exactly the

Figure 2

“HOT”

JUNCTION

Fe

Fe

CN

CN

EXTENDER

Cu

Cu

“NEW” COLD

JUNCTION

AMPLIFIER

FRONT PANEL

CONNECTOR

LT1025 • AG02

LT1025

APPLICATIO S I FOR ATIO