AD8421 datasheet(20/28 Pages) AD

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LT6372-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

generates a small temperature-dependent voltage. Also

known as the Seebeck Effect, these thermal EMFs can be

the dominant error source in low-drift circuits.

Connectors, switches, relay contacts, sockets, resistors,

and solder are all candidates for significant thermal EMF

generation. Even junctions of copper wire from different

manufacturers can generate thermal EMFs of 200nV/°C,

which is comparable to the maximum input offset voltage

drift specification of the LT6372-1. Figure 3 and Figure 4

illustrate the potential magnitude of these voltages and their

sensitivity to temperature.

In order to minimize thermocouple-induced errors, atten-

tion must be given to circuit board layout and component

selection. It is good practice to minimize the number of

junctions in the amplifier’s input and RG signal paths and

avoid connectors, sockets, switches, and relays whenever

possible. If such components are required, they should be

selected for low thermal EMF characteristics. Furthermore,

the number, type, and layout of junctions should be matched

for both inputs with respect to thermal gradients on the cir-

cuit board. Doing so may involve deliberately introducing

dummy junctions to offset unavoidable junctions.

Air currents can also lead to thermal gradients and cause

significant noise in measurement systems. It is important

to prevent airflow across sensitive circuits. Doing so will

often reduce thermocouple noise substantially. Placing

PCB input traces close together, and on an internal PCB

layer, can help minimize temperature differentials result-

ing from air currents reacting with the input trace thermal

surface area.

Figure 3.

TEMPERATURE (°C)

1.8

2.4

3.0

2.8

2.6

2.0

2.2

1.4

1.6

0.8

1.0

0.2

0.4

6370 F03

1.2

0.6

Thermal EMF Generated by Two Copper Wires

From Different Manufacturers

Figure 4. Solder-Copper Thermal EMFs

SOLDER-COPPER JUNCTION DIFFERENTIAL TEMPERATURE

SOURCE: NEW ELECTRONICS 02-06-77

6370 F04

–50

–100

100

SLOPE ≈ 1.5µV/°C

BELOW 25°C

SLOPE ≈ 160nV/°C

BELOW 25°C

64% SN/36% Pb

60% Cd/40% SN

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