LT6370 datasheet(19/28 Pages) AD

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LT6370

Rev. 0

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APPLICATIONS INFORMATION

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 4.

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 F04

1.2

0.6

Thermal EMF Generated by Two Copper Wires

From Different Manufacturers

Figure 5. Solder-Copper Thermal EMFs

SOLDER-COPPER JUNCTION DIFFERENTIAL TEMPERATURE

SOURCE: NEW ELECTRONICS 02-06-77

6370 F05

–50

–100

100

SLOPE ≈ 1.5µV/°C

BELOW 25°C

SLOPE ≈ 160nV/°C

BELOW 25°C

64% SN/36% Pb

60% Cd/40% SN

Reducing Board-Related Leakage Effects

Leakage currents can have a significant impact on sys-

tem accuracy, particularly in high temperature and high

voltage applications. Quality insulation materials should

be used, and insulating surfaces should be cleaned to

remove fluxes and other residues. For humid environ-

ments, surface coating may be necessary to provide a

moisture barrier.

Leakage into the RG pin conducts through the on-chip

feedback resistor, creating an error at the output of the

pre-amplifiers. This error is independent of gain and

degrades accuracy the most at low gains. This leakage

can be minimized by encircling the RG connections with a

guard-ring operated at a potential very close to that of the

RG pins. The DFN package has NC pins adjacent to each

RG pin which can be used to simplify the implementa-

tion of this guard-ring. These NC pins do not provide any

bias and have no internal connections. In some cases, the

guard-ring can be connected to the input voltage which

biases one diode drop below RG.

Figure 6.

6370 F06

–IN

–RG

+IN

+RG

LT6370

Guard-Rings Can Be Used to Minimize

Leakage into the RG Pins

Leakage into the input pins reacts with the source resis-

tance, creating an error directly at the input. This leakage

can be minimized by encircling the input connections with

a guard-rings operated at a potential very close to that

of the input pins. In some cases, the guard-ring can be

connected to RG which biases one diode above the input.

Figure 7.

6370 F07

–IN

–RG

+IN

+RG

LT6370

Guard-Rings Can Be Used to Minimize

Leakage into the Input Pins

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