LTC7652_15 datasheet(9/24 Pages) LINER

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LTC1052/LTC7652

1052fa

When connectors, switches, relays and/or sockets are

necessary they should be selected for low thermal EMF

activity. The same techniques of thermally balancing and

coupling the matching junctions are effective in reducing

the thermal EMF errors of these components.

Resistors are another source of thermal EMF errors.

Table 1 shows the thermal EMF generated for different

resistors. The temperature gradient across the resistor is

important, not the ambient temperature. There are two

junctions formed at each end of the resistor and if these

junctions are at the same temperature, their thermal EMFs

will cancel each other. The thermal EMF numbers are

approximate and vary with resistor value. High values give

higher thermal EMF.

APPLICATIO S I FOR ATIO

connections, to the inverting input. Guarding both sides

of the printed circuit board is required. Bulk leakage

reduction depends on the guard ring width.

Microvolts

Thermocouple effects must be considered if the LTC1052’s

ultralow drift is to be fully utilized. Any connection

of dissimilar metals forms a thermoelectric junction

producing an electric potential which varies with

temperature (Seebeck effect). As temperature sensors,

thermocouples exploit this phenomenon to produce

useful information. In low drift amplifier circuits the effect

is a primary source of error.

Connectors, switches, relay contacts, sockets, resistors,

solder, and even copper wire are all candidates for

thermal EMF generation. Junctions of copper wire from

different manufacturers can generate thermal EMFs of

200nV/°C—4 times the maximum drift specification of

the LTC1052. The copper/kovar junction, formed when

wire or printed circuit traces contact a package lead, has

a thermal EMF of approximately 35µV/°C–700 times the

maximum drift specification of the LTC1052.

Minimizing thermal EMF-induced errors is possible if

judicious attention is given to circuit board layout and

component selection. It is good practice to minimize the

number of junctions in the amplifier’s input signal path.

Avoid connectors, sockets, switches and relays where

possible. In instances where this is not possible, attempt

to balance the number and type of junctions so that

differential cancellation occurs. Doing this may involve

deliberately introducing junctions to offset unavoidable

junctions.

Figure 2 is an example of the introduction of an

unnecessary resistor to promote differential thermal

balance. Maintaining compensating junctions in close

physical proximity will keep them at the same temperature

and reduce thermal EMF errors.

–

LTC1052

LTC1052/7652 • AI03

OUTPUT

NOMINALLY UNNECESSARY

RESISTOR USED TO

THERMALLY BALANCE OTHER

INPUT RESISTOR

RESISTOR LEAD, SOLDER,

COPPER TRACE JUNCTION

LEAD WIRE/SOLDER/COPPER

TRACE JUNCTION

Figure 2

Table 1. Resistor Thermal EMF

RESISTOR TYPE

THERMAL EMF/°C GRADIENT

Tin Oxide

~mV/’C

Carbon Composition

~450µV/°C

Metal Film

~20µV/°C

Wire Wound

Evenohm

~2µV/°C

Manganin

~2µV/°C

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