5

LT1025

1025fb

same temperature, but temperature gradients exist within

IC packages and across PC boards whenever power is

dissipated. For this reason, extreme care must be used to

ensure that no temperature gradients exist in the vicinity

of the thermocouple terminations, the LT1025, or the

thermocouple amplifier. If a gradient cannot be eliminated,

leads should be positioned isothermally, especially the

pins, and the gain setting resistor leads. An effect to watch

for is amplifier offset voltage warm-up drift caused by

mismatched thermocouple materials in the wire-bond/

lead system of the IC package. This effect can be as high

as tens of microvolts in TO-5 cans with kovar leads. It has

nothing to do with the actual offset drift specification of the

amplifier and can occur in amplifiers with measured “zero”

drift. Warm-up drift is directly proportional to amplifier

power dissipation. It can be minimized by avoiding TO-5

cans, using low supply current amplifiers, and by using the

lowest possible supply voltages. Finally, it can be accom-

modated by calibrating and specifying the system after a

five minute warm-up period.

Reversing the Polarity of the 10mV/

°C Output

The LT1025 can be made to “stand on its head” to achieve

a minus 10mV/

°C output point. This is done as shown in

back is established between the ground pin and the

positive supply pin by feeding both of them with currents

while coupling them with a 6V zener. The ground pin will

now be forced by feedback to generate –10mV/

°C as long

as the grounded output is supplying a net “source” current

into ground. This condition is satisfied by selecting R1

of the LT1025 supply current, the maximum load current

R

V

AI

R

VV

V

VR

A

L

Z

1

300

2

6

1 280

=

µ+

=

≈

+µ

+

–

–

,

–(

)

/

For

R2 = 15k.

Amplifier Considerations

Thermocouple amplifiers need very low offset voltage and

drift, and fairly low bias current if an input filter is used. The

best precision bipolar amplifiers should be used for type

J, K, E, and T thermocouples which have Seebeck coeffi-

cients of 40

µV/°C to 60µV/°C. In particularly critical appli-

cations or for R and S thermocouples (6

µV/°Cto15µV/°C),

a chopper-stabilized amplifier is required. Linear Technol-

ogy offers three amplifiers specifically tailored for thermo-

couple applications. The LTKA0x is a bipolar design with

extremely low offset (< 35

µV), low drift (<1.5µV/°C), very

low bias current (<1nA), and almost negligible warm-up

drift (supply current is

≈400µA). It is very cost effective

even when compared with “jellybean” op amps with vastly

inferior specifications.

For the most demanding applications, the LTC1050 and

LTC1052 CMOS chopper-stabilized amplifiers offer 5

µV

offset and 0.05

µV/°C drift (even over the full military

temperature range). Input bias current is 30pA, and gain

is typically 30 million. These amplifiers should be used for

R and S thermocouples, especially if no offset adjust-

ments can be tolerated, or a large ambient temperature

swing is expected.

Regardless of amplifier type, it is suggested that for best

possible performance, dual-in-line (DIP) packages be

used to avoid thermocouple effects in the kovar leads of

TO-5 metal can packages if amplifier supply current ex-

ceeds 500

µA. These leads can generate both DC and AC

offset terms in the presence of thermal gradients in the

package and/or external air motion.

APPLICATIO S I FOR ATIO

Figure 3

R2

15k

R1

47k

–10mV/

°C

D1

LT1025 • AG03

LT1025

GND